Well, I know there are a lot of others who can articulate it better than I can, but I'll try and give you the basics as simply sounding as I can...
First, find out what your internal gear ratio is of your car. Every cars gearbox has a different size diff gear, idler gear, etc. and that adds up to an Internal Drive Ratio and from that you can find other things you need to know like FDR or Final Drive Ratio and Rollout. I wont even get in to rollout because it will confuse you, but the short explanation is it figures how far of a distance your call will travel on one full revolution of the wheels. It takes in to account pinion/spur size and tire diameter.
FDR will be what you'll need to find. Right HERE
is a nice chart for Losi's that explains it pretty well. Every track or certain motor combos and whatnot have a good FDR starting point to begin gearing at. Check that link, then calculate what your FDR is and see where your at. ALSO, please give more info on your car and motor and everything and you will get some much more specific info to your car.