Excellent advice in the previous post. I'd just add that, before you pick a credit card, answer two more questions for yourself:

1. Which airline(s) do you want points on? With rare exceptions, most people don't earn enough miles through credit cards to get good awards. If (unless you're one of those exceptions) you can't get miles on an airline you'll also fly on a good deal, it's not the right credit card.

2. How many $$ (or whatever) will you charge per year? That affects whether or not the miles justify the fee.

Finally, if you expect to earn enough just through charging to get awards and your primary interest is in economy-class flights, consider a card where you collect points in their own program rather than airline miles. They only make sense if that's your profile, but if it is, they can work out well.