i have good credit and won't be spending more money than i have--an i'll be paying off the full amount each month before the month ends. i just need a credit card to build up better credit. i've heard sometimes they give you free gas or something with every dollar you spend on the credit card?

A few of the better rewards cards include the American Express Blue Cash card (up to 5% in some categories, but tiered), the Citi Driver's Edge card (6% in some categories in the first year), Fidelity Signature Visa (1.5% cash back on all spending), and the Capital One No Hassle Miles Ultra for Professionals card (approx. 2% reward on all spending as airline tickets, though Capital One's reputation for customer service isn't as good as, say, American Express).

You can use this rewards calculator to see which rewards card will pay you the most for your normal spending profile:
http://www.creditcardtuneup.com/