How to connect my iPhone to a new library on a new computer?

I got a new computer recently but when I plug in my iPhone, it syncs but nothing else, I can't put music on my phone or anything, any help?

Added (1). I didn't get an "erase and sync" message when I first plugged it in.

Here is an article from apple that explains what to do:

http://support.apple.com/.../kb/HT4527