I just confuse about kanji. I just learn a bit of kanji. But I want to know when exactly we use it? is there a rule about that?
Up to a certain point, the basic rule of thumb is to use it whenever you can. In other words, if you are writing a word and there's a kanji for it, always prefer kanji. When I say "up to a certain point", I'm thinking about 常用漢字, 2000+ letter set designated as a common kanji character set for everyday use. From the sound of what you are saying, you are well within this bound, so you should just use as much as you can.
I could have stopped here, but when I asked myself "how do I choose when to use kanjis?", it got more interesting.
Unless you are writing to kids, I think the basic rule of thumb still applies, even to native Japanese. I can't recall any occasion where I thought "I know the kanji but let's stick to hiragana." In fact the stigma of using hiragana where one should be using kanji is so strong that when you know you should be using kanjis but can't remember how to write it, you see people writing it in katakana, to indicate that those are supposed to be kanjis (this also has a practical benefit of assisting readers, as all-hiragana text would be very hard to read.)
I should also note that there are some kanji expressions that are disappearing, such as 有難う. While both kanjis in this is in 常用漢字, it's more common now to write this as ありがとう. The former expression is far from wrong, even correct in some context, but it sounds more formal and rigid. If I think more, I can probably come up with a few more.