You'll need to unscrew all the screws/bolts holding the plate unto the car. Typically there are three types of screws/bolts used to do this, you'll have to look to see which applies to your situation- screwdriver type (either flat head or phillip's), allen wrench type, or hex head.
Some times you have the security bolts that require a special tool to remove them. You can find these tools at any auto supply store if that's the case.
If you are serious, get yourself a medium phillips head screwdriver. Place the screwdriver on the head of the screw holding the license plate on to the vehicle, and turn counter-clockwise...(if it's rusty, you might have to use some serious muscle). Continue turning until the screw falls out. Repeat for the other screws. Take off the plate.