I know a girl from my friends , I added her on fb and I started messaging " thank u 4 the accept or sth like that " I used to message her on fb and talk about university and natural things , until that one day I asked her on fb how is she doing? and she didn't respond to me , that was a week ago , now I want to message her and say " sorry but i don't want to be creepy or annoying , but do you mind if i attend a class with you in the univ. ", is that a good behavior from me ? Am I creepy ? Shall I send her this message ?

No, you don't jump from friendly messages on Facebook to asking if you can take a class together - that's much too much.
And especially now that you haven't heard from her in a week... she may be busy, she may have a boyfriend, she may have decided she doesn't like you enough to keep talking. I'd give it another 4 days or so, write one more little friendly 'hi how's school' and drop the whole thing if she still doesn't respond.

What you're telling us here may or may not be exactly as you talked to her. You say you talked about "natural things", and that you asked her how she is doing... depending on the actual wording you used to talk to her, it might have come off as creepy to her. Depending on how often you wrote to her, sure, it could have become annoying as well.

Back off, let it go for a while... and definitely don't be asking to take classes together.