The first step in getting rid of roaches is removing their food source. Sweep and mop all floors regularly, and keep all sources of food covered, bagged, or otherwise inaccessible. If you've got roaches in your car, vacuum the carpets (especially under the seats) to remove any crumbs which are hidden below. Removing the back seat cushion (usually by pushing it back and upwards) and searching for dropped French fries and small fortunes in missing coins is also a viable option. This alone may get rid of your roach problem.
If you've still got the annoying bugs around, it may be time to take some additional steps. I prefer using non-chemical means of pest control, so I usually pick up a few Roach Motels. The Roach Motel is a glue trap, which roaches and other insects get trapped inside after being attracted by a pheromone attractant. Place Roach Motels or other glue traps near where you usually see roaches enter or leave a room. (Closing off this entrance/exit, if it's an unwanted hole, is also a good idea.) Discard and replace the trap when it's full.
If trapping the roaches doesn't solve the problem, it may be necessary to set out poison baits. These are usually sealed inside a plastic disk to make them accessible only to roaches, but not to humans or other animals. The roaches will supposedly consume some of this, and take some back to their nest, eradicating the whole bunch. Of course, always take these marketing claims with a grain of salt. They do seem to greatly reduce the number of roaches running around, though.
There are other methods of removing roaches, which are best left for application by a professional exterminator. These include the placement of a bait compound inside walls, crawl spaces, and spraying.
Of course, stomping or otherwise squishing roaches is always helpful, as long as their gooey remains are cleaned up.