Is it just .... add credit card and start buying or they put a hold on your card to verify it?
Add your credit card to your iTunes.
When you make a purchase on iTunes, if I'm not mistaken they follow a cycle before they bill you the total amount of purchase you've made within a time frame.
When that happens, you'll receive an email showing all of your purchases within that certain time frame/cycle, plus tax then that's what gets charged on your credit card. You could review your purchases on iTunes, just sign-in using your account and on the upper right hand section, click your username or email address.
Of course they would post a "memo" on your credit card (you might see this as a pending), that guarantees iTunes will be paid, in turn on your credit card will bill you after their billing cycle.