Are you in the USA?
If so, you'll pay for the phone ($199 or $299 + tax), assuming you're a new AT&T customer, and a deposit if AT&T requires one. You'll be billed for everything else.
In your first bill, you'll be charged an activation fee of $36, a pro-rated amount for your first month of service, the next month in advance, and taxes. Your second bill will just be for any overages, and the regular month's fees and taxes for the coming month.