Welcome to the Support Communities!
Now that you have your Apple ID created, the links below will provide information on how to enter or update your billing information. I've also included some general information on the iTunes Store for you.
Once iTunes is installed, click on the Store command on the menu bar, and sign in to your account using your Apple ID and password.
Do you have a copy of your Music folder from your old computer? You can add your music to your new iTunes library: