You'll have to redownload everything. There is no automatic downloading of previous purchases to iTunes on a new Mac. Keep in mind that there are a limit of 5 authorized computers so you need to deauthorize iTunes on your old computer or you'll eventually run out unless you're still using it. I'm figuring that computer doesn't presently exist or you'd know which ones you had downloaded.
As for what you previously downloaded, open iTunes and go to the iTunes store. Go to the "Quick Links" on the right and select Account. Log in and, at the next screen, select Purchase History. You'll get a page that has a list of all your downloads.