Sprockets

How many teeth?

Ummm, well it's not rocket science - the more teeth you have on your sprocket, the lower the gear ratio, and the faster you have to pedal to go at a given speed! If you are a fast cyclist intending to use your bike for flat-out training rides on empty roads at top speed (or on the track), you want a smallish cog.

On the other hand, if you're intending to use the bike for knocking around town, stopping and starting a lot, and maybe have a few hills around, then it's far better to start off with something with a few more teeth. This will have the advantage of teaching you to increase your cadence when you are on the flat straights anyway!

It's a quick job to swap sprockets over to change the gear ratios (even easier if you have a double fixed hub - then it's a matter of seconds to take the wheel out and spin it over!), so you could always get a couple to ensure you always have the right ratio*

*although if changing gear ratio is really important to you, you might want to consider one of those new-fangled deraillieur thingys