You would use 不. You're referring to the reply to: "How are you doing?", right?

You would say "不太好". If you wanted to lighten the mood , you'd still use "不", but it would be changed to "不错", which means "not bad". Using "没" is different. It's more like "haven't".

我 不 好。

you can say'‘ 我不是很好’

Translated into is我不够好，Do you like Chinese ?