Informal language is a little different, and it entirely depends on your meaning. The difference between formal and informal, to me, is the difference between the way you would speak to your boss and the way you would speak to your friends. If you meet a friend in the street and say "Hiya. How's it going?" that's perfectly normal, but with your boss, you're more likely to say "Good morning. How are you?"
I certainly wouldn't expect you to say "Good morning. How are you?" when you meet a friend, so in that example using "informal" language while you're still a relative beginner is fine.
Slang is different and can be quite hard to master. If you use it in the wrong way, people might not understand what you're trying to say and you may end up feeling silly.
By the way, I've never heard of that book - can't you borrow it from the library and take a look? If you think it's useful, then great. If not, take it back!
Student or Learner