First, a few corrections: if you're going to say "I think [it's] bad" and use 悪い, you don't use だ, so you'd say 悪いと思います. But here 私の日本語はまだ上手じゃありません ("My Japanese still isn't good.") or 私の日本語は下手です ("My Japanese is bad."). You got the reading s right for いちもん, but the second character is missing a mouth: it should be 一問. Also, the "want to" form of 聞く is ききたい, but I'd probably say ちょっと訊きたいことがあるんですが. I used 訊 instead of 聞 beacause it has a meaning of "inquire", where 聞 also means "listen" and "hear", and I feel it makes things less ambiguous, but it's not a mistake to use 聞.
As far as 思う and 思っている go, generally speaking since the first one only points to one instance, one point in time, it can only being used for something that you think or feel, whereas the second one points to a state or ongoing action, so it can be used for things you are thinking or feeling and can also be used for things other people are thinking or feeling. So you can say (私は)これがいいと思う, but you can't say 彼はこれがいいと思う. You can, however, say 彼はこれがいいと思っている. Does that make sense? I hope it's not too much all at once.