It depends on how complex you want to get with it. You could try to get one of those dorm fridges (the biggest kind you can get) as one of these could hold a pony keg. You'll probably want to buy your own tap. If you can, go with a gas tap instead of an air tap as the beer will stay fresher longer. Get all the hoses, the tap, and the connectors that you will need. Drill holes in the door and in the sides as necessary, run your hoses, and seal the edges of the holes around the hoses with aquarium grade silicone (available at some pet stores and aquarium stores).
Otherwise, you are going to look at buying a much larger fridge. If you can get a full-size fridge used at a good price, you can do the same thing as described above and have enough room for a full sized keg.
Ryan