First, try visiting your favorite airline's website. For example, I have the Jet Blue American Express card because I can get points on things I buy, plus points for travel and it has cheap routes between the two places I go to the most (San Francisco and New York City). Each dollar I spend with the card equals a point, and after 100 points, you get one TrueBlue Point. Then, 100 TrueBlue points equal two one-way plane tickets. Now that may seem difficult to achieve if you're only spending money on the card, but flights across country come out to 24 points if you book online. Thus, after four flights and at least $100 spend on the card, you get a free plane ticket. Not a bad program.
If you don't have a favorite airline (and these days, with air travel as hectic as it is, I don't blame you), you can find an airline within a large network (i.e. OneWorld) and find a credit card from one of those airlines. Typically, these airlines allow you to use your rewards points on any carrier within the system. Frequent flyers with a British Airways Visa Signature card, for example, can use their rewards miles on tickets with American Airlines and Alaska Airlines as well, thus having more flexibility when booking travel plans.
Another option is checking out your bank's website for credit card offers and deals. However, I find this path to be confusing, as banks tend to inundate you with choices, making it hard to decipher and decide which option works best for you.
Besides the point system (and, as with any credit card, the interest rate) a key thing to keep in mind when choosing a credit card: the annual fee. You don't want to end up with an annual fee that approaches the rate of a plane ticket, or you're not really getting a good deal. Some cards, like the United Mileage Plus Visa card, don't charge an annual fee the first year. However, their fares tend to be higher to begin with. Southwest, which has prided itself as a discount American carrier, charges an annual fee of $59 on the Southwest Rapid Rewards Visa card, but $59 is probably not bad for a round-trip ticket from coast to coast.