From my experience, everyone is really nice.
Since everyone is studying a language themselves, I think they all know and respect the fact that everyone starts out bad, so I really don't think you have anything to worry about. I've not actually tried myself, but I'm sure no one would mind if you posted completely in hiragana/katakana. Maybe even in romaji, who knows. If you still feel uncomfortable, you could correct other people's English until you think you're ready to write something of your own.
Come on, join the dark side!
Everybody on that site is super-super-nice, I've not found any mean people yet. (If anything, they're too
friendly! I've gotten over a dozen friend requests in a week, many from people I made just one correction to, and I'm afraid to deny any of them because I don't know if it will tell them I denied them!
) It is fine if you write in hiragana, people will correct it with hiragana or suggest kanji you can use.
Like hinikuya says, maybe you can just correct people's English until you feel more comfortable.
Mami chibi drawn by Jiino, animated by me. | Sophie sig by Misuzu | My Anime-Planet page
but you know, not every image has a source and not every character has a name :3