Open iTunes, and sign into the iTunes store using the Apple ID used to purchase your music (Store>Sign In). Also authorize the computer to play your purchased music (Store>Authorize This Computer). Then download your previous purchased as explained here: http://support.apple.com/kb/HT2519. Note: You can only authorize 5 computers to download previous purchases for your ID. If you have done so already, you should deauthorize your old computer by opening iTunes on it and going to Store>Deauthorize This Computer and signing in with your ID. If you no longer have access to your old computer you can deauthorize all computer once annually as explained here: http://support.apple.com/kb/HT1420.