Let's say you're on the road, unabashedly singing along to a Madonna song on the radio, when you suddenly realize you've got a flat tire. Do you get teary eyed and call AAA/your boyfriend/your cousin who took auto shop in high school? No. This is the time to prove your real life womanly abilities — and there's no crying in automotive repair.
The first thing to do if you have a flat is pull of the road to a safe place. Note: safe place. It seems obvious, but hundreds of people are killed every year while trying to do curbside car repairs. Driving an extra 100 feet isn't going to hurt the car as much as getting hit by a truck, so get far away from traffic and try to find stable ground.
Put the car in park, set the parking brake, and turn on your hazard lights. Wedge logs or bricks behind the three good tires just to make sure the car won't roll. In the trunk you should have a spare tire, a jack, and a tire iron. Make sure you have these in your car today. If you can remember to toss your gym bag in the passenger seat every morning, you can take the time to throw some tools in your trunk.
First, remove the hubcap and loosen the lug nuts, but don't remove them completely. Turn the nuts counterclockwise (you won't look as cool if you have to say "righty tighty, lefty loosey.") You may need to put your weight on the wrench (sorry, skinny girls, this is one time carrying a few extra pounds pays off).
Place the jack under the metal frame of the car, next to the wheel you are going to replace. Important: this is not the edge of the car, but the sturdy metal part underneath. Despite the shiny metallic finish, many cars actually have a plastic exterior that can crack like an Easter egg. If you're not sure where to put the jack on your car, it will say in the owner's manual. You should read it now, so you're not trying to make sense of it at 3 a.m. on the side of the highway.
Raise the car up a few inches higher than the tire so there is room to fit the spare on. Next, completely unscrew the lug nuts and remove the flat tire. Place the old tire under the car, so if the jack fails, the car will land on the tire, not you. Remember to keep track of the lug nuts, since this will be when one rolls off into the street or falls in the mud.
Put on the new tire and replace the lug nuts, using a cross pattern so you're not tightening adjacent screws. Lower the jack and put the bad tire in your trunk. Your mechanic may be able to repair the tire, and if not he will know how to properly dispose it.
You're sure to get major macho points for replacing your own flat tire, but don't blow it by continuing to drive on the spare. Just cough up the $100 for a new tire. Spare tires are not meant to be driven for long distances, and most are not rated for more than 50 mph. After you change your flat, make your next stop an auto shop.