You car's brake rotors are among the easiest parts to rust because of their dark, dank location. Even common car care like washing will cause rust to quickly build up. It's no problem to remove rust that can interfere with the brake operation. Any rust along the edges, however, can be more difficult, depending on how much rust has developed.
Apply the brakes while driving. This is really all you should need to do; while the rust can easily build up, it will come off when the rotor rubs against the pads. You can slam on the brakes once or twice to remove excessive rust, but make sure you're in an empty lot where it's safe.
Clean the brakes with an aerosol cleaner, using a dripping pan to catch all the liquid residue, and wipe the rotor clean with a rag. Brake cleaner works best, since it also evaporates quickly. Don't use oil like WD-40 unless you can wipe all of it off, since lubricated brakes won't work very well.
Scrub the rotors with something abrasive if any rust remains. Apply the brake cleaner again, and scrub the rusted area thoroughly with steel wool or a wire brush.
Use a liquid cleaning agent on excessive rust. The cleaning agent usually comes in concentrate form and must be mixed with a gallon or so of water. This is the most difficult option since you'll need to disconnect the rotor from the car and submerge it in a container of the solution.
You must remove the wheel before you can access the brake rotor. Raise the appropriate car end on a jack and use the car's lug wrench to disconnect the wheel's lug nuts. To remove the rotor, disconnect the brake caliper and its mounting bracket from the rotor by removing their bolts; this can require a ratchet and/or hex wrench, depending on the model. Then pull the rotor right off the studs; this might take effort if there is excessive rust.