I would like to know what terrapin to get (map, yellow belly etc) I was wondering which one is the smallest and what sized tank and generally what stuff is needed for terrapins (filter etc) thanks

Common Musks are the smallest growing to 3-4 inches as adults and Razorback Musks tend to be 4-5 inches full grown...... African Helmeteds grow to around 6 inches...... Whatever you do, avoid slider species as they can grow huge.... Males at 6 inches and females upto 12 inches.....
You need a tank that holds 40 litres/10 gallons per shell carapace length so for a full grown musk, you'd need a 40 gallon aquarium....... Any questions just ask, I keep loads of turtles......
You also need a UV-B light (any others are usually too weak or will kill them), you need a heater and a very strong filter. External ones are best, the Eheim 2215 or 2217......