If you pay the balance of the credit card off in full right away, usually within 30 days of the statement coming out, or even BEFORE your statement comes out, then you shouldn't get hit with any interest charges. Read the fine print with the credit card you get to be sure.
Also: Look into online bill payment with your bank. Most people like to pay their bills online at the website of the credit card issuer (and at AT&T's website for their phone bill), but these people ignore the fact that they get a LOT more control over their bills if they use the (usually free) online bill payment service their own bank gives them. You just go to one website, you're not giving out your credit card number all over the place, and you decide exactly on what date the bill gets paid, not the credit card company, and not AT&T.