Choose what you want to race, and the classes available at your local track. Then check out the classes they run.

You can pick up late model rollers (cars minus electronics) for a few hundred dollars and then buy electrics to suit the category you want to run (stock, mod, etc.). Alternatively there are RTR (ready to run) packages which will come with electronics and controllers, etc. for ~$500. They're a quick and easy way to get in and are usually similarly built to popular models from the manufacturer, so there will be hop up parts available.