Buying the Right Gears
If you are in the market for a new bicycle, check out my previous post, Cycling Tip of the Week: Choose the Right Kind of Bicycle, for some tips. That will give you a sense of what kind of bike—hybrid, racing, mountain, etc.—is right for you, but it won't help you with the right kind of gearing. Unless you purchase a single-speed or "fixxie" bike (bikes with only one gear), you will want your choice of gearing to be a top priority.
First and foremost, ask yourself what kind of riding you will be doing. If you never plan on riding up or down hills, then a bike with 1 to 7 speeds will be more than sufficient. However, if your rides will include any kind of incline, then good gears will make life a lot easier. The most important thing to consider is gear range; that is, you want to be able to get into as low a gear as possible for going uphill, and you want to get into as high a gear as possible for going back down. Typically, a triple chain ring on the front of the bike gives you a good range of gearing, with a 8, 9 or 10 speed cassette on the back (I know, the lingo might be a little confusing, but it's good to know the terms when you are in a bike shop). The chain rings are the circles with teeth on top of them that the crank is attached to. The cassette refers to the circles with teeth that is attached to the rear wheel, and that the chain engages when you pedal.
Aside from gear ratio, the other important consideration is gear quality. High-end gears, such as Shimano Dura Ace and Campagnolo Record, are extremely light and smooth, but there are lots of affordable, reliable options. For racing, I recommend at least a Shimano 105 group (group refers to the chain, front and rear derailleur, cassette, cranks, chain rings, bottom bracket, brakes, brake levers and "hoods"). But for commuting, there are plenty of groups that will work just fine.
If you own or buy an old bike, the first thing you should do is test out the gears and replace the parts, such as the derailleur, that no longer work. It's amazing how a 30 year old frame can ride like a dream provided you spend $50-$100 to replace a few parts. A new chain and cassette, for instance, can dramatically improve shifting performance.