What is a good website to write stories, post them up and get feedback?

There are many sites for posting (FictionPress, FicWad, Booksie, and so forth). If you want feedback, though, you're going to have to earn it.

Post something you've really put your heart into, but before that, get someone to edit it -- look up "beta reader" and you should be able to find one for free. Comment on other people's stories. Post on forums. If you're sociable and write, well, (a better love story than Twilight) anything decent, you'll get your feedback.

A warning: No one is born a great writer. Few published works bear significant resemblance to their first drafts. Some of the feedback you recieve may well be negative. Please don't be hasty with accusations of flaming (though anyone labeling you a homosexual without context is probably safe to ignore). Put yourself in the reviewer's shoes: under what circumstances would you write "The plot sucks and the characters are boring"? That's probably about the situation your drive-by critic was in. Look over your story: is the problem the critic suggested reparable? Is it something you can do better next time? Is it something you need an editor for? As twelve-step programs advertise, you should never be ashamed to need help.

The immortal Professor Strunk once said, "Omit needless words." Say what you mean to say, and no beating around the bush; write well, and write clearly; they will come.