I'm not going to restate the needs for a community that will encourage and criticize your work per need. The reasons for joining such a group should be obvious. Admittedly, finding such a place on the internet may be difficult, which is why there are three websites you should know about. They are fairly popular, so you may know of them already.
The first may seem ridiculous, a bit, because it's fanfiction.net. And if you are a person who expects to make money, you may wonder why it's worth your time to bother with writing something that will at best only get several positive reviews by people who tend to be very undiscriminating, or else very pretentious. And you may be considered highly nerdy for fanfiction.
Though viewed as pathetic, for the vast majority of people who will write, I feel that it is highly necessary, mainly because of one truth: when you start writing, you are a horrible writer. It doesn't matter how many books you have read; while reading a lot is very influencial on your style and mentality, it doesn't make you good. It doesn't matter if you're a good poet; there is a huge style difference between writing a novel and writing a rhythmical emotional encapsulation. Besides that, coming from a poetic background tends to make the writer pretentious. It doesn't matter if you have a message to say; you're still a terrible writer if you don't know how to say it in a way that conveys exactly how you feel. There's only one way to become good at writing: to write.
That's the advantage of working with fanfiction at first. Nobody expects you to be great. You can write more and more, but even if you aren't the best, this surprises no one. And if someone gives you a bad review you can just think to yourself that they are just as new as you, and they have no right to criticize. They may very well be right, but this will help overly sensitive people who need to learn to get over rejection. There is a lot of literature out there in the novel writing world, so you'll have to learn to stand out if you want to get published. And there will be rejection. Why not get your noobishness over with in front of those who don't know you?
Also, once you become an established writer and start putting out pieces that people like, you can mention a professional publication on your profile to direct people to your novel.
Another advantage, the most important one in my opinion, is that you work with pre-established characters, and if you dare take a character in a lame or incorrect direction, there are plenty of rabid fans to let you know that you are breaking character. Every character you write about has a pre-established nature. In given situations, they will do certain things, even if it means they won't do what you want them to do.
Perhaps you could do an alternate universe where a good character is bad, but the thing is, that character is still that person. If you want to write a story where, say, Ben Franklin is somehow evil, then do so. Be aware, though, that good authors will remember that even if Ben is evil, he's still Ben Franklin. He will be evil in his own, unique way. You can change certain things about characters, but you can't change their base nature without giving them a life-changing situation to go through. You can't make Ben Franklin hate inventing things, even if he's evil. He'll simply create evil things. However, it is acceptable to create a story where a scarring incident where someone is hurt by an invention of his makes Ben want to give it up. Part of being an author is knowing what about a character you can and can't change.
You might ask, what does this have to do with creating original characters from your own imagination? You see, the thing is, once you create a character and establish them well, you can't change their characterization. They have their own personality. Even though you are the one most familiar with their personality, and you know the hidden aspects of that character, that character must be consistent if you are to be a good writer. Only bad writers change their creations' personalities, attitudes and opinions without giving them a good reason for changing. You can't suddenly change them on a whim. It's one of the first signs of a bad writer.
Take for example the writer of the Drizzt series. He changes the nature of characters at his own whims, making them fall for someone and then twist that love unexpected so that the character love someone else...worse still, he injects weird quips where Drizzt is philosophizing, and this comes across as extremely pretentious. But that's another topic.
So basically, if you can keep someone else's characters consistent, you can keep your own consistent. It's good practice. But if you choose to write on fanfiction.net, make sure that you review other people's works. Other people are trying to write well also, or their just having fun writing stories that they think lead the characters interesting ways. Either way, they like reviews.
Okay, I have to make a disclaimer here. It's going to sound very weird, but unfortunately it's the truth of fanfiction.net (and fanfiction in general). In most of the Japanese originating sections (like Sailor Moon, Mega Man, Azumanga Daioh), you'll find that most people are far more interested in pairings than good writing. I've come across a few good writers in the Mega Man section, but for the most part people in these sections will write romances. The trouble is, most of them are not good at it, and when you write pairings too much, your readers will ignore bad plot or bad grammar and so on for the simple sake of the pairing. Thus, your reviewers in these sections will not be critical when they need to be to help you learn to write better.
Also, the trouble with pairings is that most people like to pair characters that would never actually date according to the canon of the story they come from. For example, in the Azumanga Daioh universe, people commonly pair up Chiyo-chan and Osaka. Chiyo is the cute girl who has skipped grades and is in high school very young, and Osaka is the ditzy, lost-in-space girl who has trouble focusing on anything. The trouble is, these characters don't have the nature to date each other. Chiyo is far too innocent for even straight relationships and she isn't the type to be gay. Osaka doesn't really seem to regard dating often at all, and is in fact the only one completely immune to the childlike cuteness that Chiyo has. Everyone else thought that Chiyo was adorable in her penguin costume, but Osaka is oblivious to this, regarding Chiyo as just a friend. She's too much in her own world to really notice too much about others.
So you can see that overactive pairings completely ignore the true nature of characters, thus eroding any true skill that the author has. Characterization is the number one key of any story, because by it people are endeared to your story. Your plot can be about anything in the world, but if your readers hate your lead character, well, that's the end of that.
Sadly, this phenomenon of pairing seems to be very common in Japanese sections, but it by no means only happens there. The Dr. Who section is getting a lot of pairing nonsense these days. That doesn't mean you won't have good writers and reviewers in those sections, but you'll be better off sticking to a universe without pairings. Try to avoid ones with too many female characters. I hate saying that (I am a girl) but the tendency of most fictitious works with lots of women is to be about sex or relationships in general, ignoring every single other thing that makes a book, movie, or comic good. I'll go into that more on another day, but for now I recommend sections like Starcraft -- dark pieces with lots of male characters and an intruiging plotline. Try to develop characters that are interesting by themselves, not just interesting because of relationships.
A quick note on pairings. You can write them if you want, but this blog is generally aimed at those who want to get published or become a better writer. Well, doing pairings won't help you there. You might get better at romances, but at the end of the day, randomly coupling your favorite book/movie/game characters will not bring you much success. It's juvenile at best.
If you're not really interested in writing fanfiction, you can always try fictionpress.com, which is their sister site for writing original work. The only trouble with this is that interest in finding stories is harder to come by. You see, people will want to read fanfictions based on their favorite story universes: Starcraft, Lord of the Rings, Sailor Moon, etc. But let's say you're writing a romance story on fictionpress. There are a lot of romance stories out there, and you will have to work harder to make your story appealing to others, especially when it comes to story descriptions: you need to make a better first impression.
But naturally if you get to know people and take an interest in what they write, then they will of course want to read you. So this would be good as an exercise in the thing that isn't always thought of by amateur writers, but is also important: salesmanship. You have to make people think that you write well, so that they will be interested enough to continue to read your work. You have to make people like your writing and want more of it.
So in the end, either one of these, or both, if you feel like you can write that much, will help you out a lot. Another good website is NaNoWriMo.com, which is National Novel Writing Month's website. Every November people around the world attempt to write 50,000 words in one month. Even though there are no prizes for winning, for some reason it's a good way to get people to write. It's a good motivation, simply to see if you can do it. Not unlike a dare.
The best part of this website is that you don't actually have to write anything superb, you just have to write. Sitting down and writing is the hardest part of writing (unless you're a shy person that has a hard time with salesmanship). You can always go back and write more later, so the temptation is to continue to put it off. You have to set standards for yourself on writing and follow them. Like, write for ten minutes a day, or an hour a day, or write five pages a day, or maybe just one page. Just get writing!