After you set up an Apple account (or if you’re using an existing AOL account instead), you can sign in to the iTunes Store to buy music and video, view or change your account information, and see your purchase history.
When you finish using the store, you can sign out to prevent anyone else who has access to your computer from making purchases using your account.
To sign in to or out of the iTunes Store:
In iTunes, choose Store > Sign In (or Sign Out).