You should sign into iCloud on your Mac first. Go to System Preference - iCloud. After you sign in, open iTunes. Log in to your iTunes Account. You can either download the Apps from iTunes that you have purchased. Or move them from your old Mac to the new one. I used a jump drive to move mine. Since the Apps have your Apple ID they shouldn't be any problems. The next thing you should do is authorize that computer to use your Apps and any apple downloads. You can also do that in iTunes.