The big and only one I really know is Scribophile, which has both free and premium memberships. You won’t be able to post right away as you earn karma points by critiquing other people’s work so that you can post your own, however this is a good thing because learning critique is important.
Forums like the ones on AbsoluteWrite might be helpful.
I would caution against posting anything you plan on submitting for publication online, because that’s essentially you publishing it. Remember, the internet is a great place but anything you put up here is forever.
I would also suggest looking around for local writing groups you might join in your own area. Those can be hit or miss, but very rewarding if you find ones with good people. They tend to be weekly meetings, you can either find them online or through bulletin boards at your local bookstores, college campuses, or libraries. I’d also suggest talking to those same librarians to see if they can point you in the direction of any creative writing programs or seminars held at those libraries which you might find helpful. A lot of cool stuff happens in libraries, so don’t forget about the real world.
If any of our followers have suggestions, we’d love to hear about them.