I put my iTunes gift card in but it won't let me bye any music

By 'doesn't let me buy music' you mean ... ? If you are being asked for your credit card details then that doesn't necessarily mean that the card will be charged (unless you are trying to gift music) - your balance should be used up first, before any remainder is charged to your credit card. If you do enter credit card details you should be able to remove it.

And if it's a new account, and you the isntructions on this page weren't followed when creating the account : http://support.apple.com/kb/HT2534, then credit card details will need to be entered before the account can be used to download any item from the store.