It has been discussed. Search here, and google is your buddy.
Are you using iTunes? If so, there are options. First you need to make sure your itunes music is non-drm'd. Android can't play iTunes old "fairplay" stuff. You can get isyncer (paid app from market) and it will do it. The app comes with directions. That's what I used, and it worked as advertised. I haven't tried it, but a lot of people also like doubletwist. I heard it was really slow.
There are other ways to do it, too, but I can't recall otomh what they are. It's not hard, if I can do it.