First, you'll want to know the difference between 5A, 5B, 7A and so on. In one of my drum stick lessons, I cover the minor variations among the drum sticks and how they change the feel of the stick. I breakdown each sticks size by number and letter so you can easily know what to look for the next time you choose drumsticks. You can learn about Drum Stick Sizes here.
Each part of the stick has a different effect on the playability and sound produced. If you're not familiar with the parts of the drum stick (taper, butt, shaft, shoulder, tip), then this lesson on How To Choose Drum Sticks will teach you the different parts and how they can alter sound.
The style of music you play should influence your decision to some degree. But, you should also allow room for creativity and not let the genre determine which sticks you choose. In general, for softer music like jazz, swing and other "lounge" types of music, you'll probably want to choose thin, light sticks like a 7A or 7B. For louder applications, like rock, country, pop, etc., choose a heavier weight like the 5A or 5B. The 5A and 5B are also recommended for beginner players. For those of you just beginning to play, click here to learn How To Play The Drums.