Three thoughts:

First, identify your redemption objectives. Do you want domestic economy class tickets for weekend trips, or do you want intercontinental business class tickets for a grand trip to Europe or Asia? Your accumulation stategy will be very different to achieve one versus the other.

Second, don't confuse number of miles or points with value. Beginners typically want to earn as many miles or points as possible without understanding that they are not all of comparable value. After you have identified your redemption objective it's straightforward to evaluate an earning opportunity by calculating the percentage it will move the needle toward that reward. A credit card that earns six points per dollar isn't better than one which earns one point per dollar if those points don't move you closer to your goal.

Flexibility adds value. Most airlines belong to one of three alliances: OneWorld, SkyTeam or Star. In general you need to belong to only one airline frequent flyer program per alliance because you can credit all of your flights within an alliance to one program, and you can redeem through one program for travel on any member of the same alliance. However, this does not mean that miles can be transferred from one alliance member's program to another. For credit card spending I would look at Starwood Preferred Guest or American Express Membership Rewards rather than a card tied to a single airline, but you need to verify that the card you choose can transfer to the airline program you choose. (Starwood, for example, does not transfer at a sensible rate to Continental or United Airlines, while Membership Rewards does not transfer to American Airlines.)