I use just short form when I talk with my friends."〜くない" Of course with people who are older than me or meet for the first time ,I use desu/masu form. In my opinion, "〜くありません"is used in more formal place like public place ?? So I usually use "〜くないです"or when I want to say more carefully,I use”〜くないと思います。”

That's easy. Pay attention to which form your friend is using and you should do the same. :) If you don't like that answer, I would suggest as a rule of thumb that you stick to the desu/masu form for the first couple times after you make friends with somebody and then move on to the informal form if the other person seems happy with it. Using the informal form right away can make you sound a little rude but sticking to the formal form for too long can sound distant.

Talk to friends in the short form. The "masu" form sounds silly when talking to friends.

Yeah, that can be quite puzzling issue for a foreigner, when to stop using です/ます　forms in conversations with friends. Another thing is, as far as I'm concerned Japanese people don't use くありません form too much. They rather say くないです.