Most vehicle wheels are made of steel or aluminium. Aluminium is lighter, more attractive, won't rust, and offers better handling and performance than steel, but it's also more expensive. Steel wheels have an advantage in that, if bent, they usually can be straightened. And if you can't repair them, they're cheaper to replace. Surface damage to aluminium wheels can be more easily repaired. You can repair damage to both types of wheels yourself in a few steps.
Jack up your car and remove the wheel if you have a bent steel rim. Many auto repair shops (at least reputable ones) won't fix steel rims, noting that new replacement wheels are cheaper.
Depress the valve pin to remove air from the tire. Remove the valve stem using a valve-stem remover or a pair of pliers. Don't reuse the valve stem. Instead, buy a replacement.
Lay the tire/wheel flat on the ground. Step on each side of the outer edges of the tire. Bounce up and down, or even jump, to break the tire/wheel bead.
Remove the tire from the rim by inserting the handle-end of a pair of pliers. Don't use a screwdriver or similar sharp tool that can damage the innertube (unless you'll be replacing the tube anyway).
Grease the pliers handle with vaseline or another lubricant and scroll around the inside of the tire along the rim, popping the tire over the rim edge. Force the other edge of the tire over the same rim edge.
Inspect the damage. A steel rim bent at the outer edge---usually after hitting a pothole or curb---can be repaired fairly easily. Damage to the inner part of the wheel, near the hub, can be trickier, or even dangerous, to repair. Get a professional opinion about the degree of damage if you're unsure. Aluminium wheels, while they can be repaired, are prone to developing cracks if the bend is severe.
Place the wheel flat on a heat-resistant surface, such as a concrete garage floor, or secure the wheel in a vice. Using a propane torch or blowtorch, heat the damaged area of the rim. Again, this is a simple matter with steel wheels, but aluminium wheels can discolour when heated, especially alloy wheels containing magnesium. It's best to have a professional repair an aluminium wheel.
Heat the steel for about a minute. The rim will become a flat, almost flaky, colour. Depending on the severity and location of the damage, use a hammer, channels locks or pliers to bend the edge of the rim back into shape. You might have to start with a hammer, get the rim close to its original shape, reheat the rim, and then finish the process with pliers.
Let the rim cool, then sand the repaired area to remove burrs. Inspect the wheel for any cracks that may have developed during repair.
Despite the booming aluminium- and custom-wheel aftermarket, steel wheels are making a comeback. Many low-end aluminium-wheel customers are turning to steel when replacements are needed due to lower cost and improvements in steel stamping techniques, which allow for more attractive steel-wheel models. (See References 3) Aluminium wheels that suffer surface damage, like rim scratches from curbs, can be fixed inexpensively with various home remedies. (See References 4)
Fixing damaged aluminium wheels requires very specific heat, consistent hydraulic pressure and light vibrations under pressure. Cracks undetectable by the naked eye, such as those hidden by chrome or paint, will pop out and need to be filled and repaired.