There are a lot of ways that cars get dented, even if you're a responsible driver. Small dents can be caused by runaway shopping carts, incorrectly thrown footballs and the like; door dings can occur when you open your car door into another vehicle or a barrier; and deep dents often can't be fixed without significant bodywork at a repair shop.
If your car fender is made from sturdy thermoplastic and you can access the inside of the dent, you can often pull it out yourself. Pour boiling water over the area to soften it, then reach behind your car fender and push gently but firmly on the dent. This method can work on shallow or deep dents, and is especially effective on plastic bumpers and fenders. (However, you can only heat thermoplastics so many times before they deform and discolor permanently.)
If your car has a large or deep dent in its fender, you may need to use a professional dent-remover tool ($40 or less) to get it out. These kits typically come with a bridge-type tool that straddles the dent and a variety of pulling attachments to pull out your dent. Apply the glue from the kit to the deepest part of your dent and attach the pulling head. Once it's attached, you should be able to pull on the head to suction the metal outward. Be aware that this process might create new dents that you need to smooth with your hands or the tool.