Something discrete that can easily be covered, no big deal. So upper arms, legs, torso and back shouldn't get you in any serious trouble- it's not like your boss can order you to work shirtless or to wear shorts, if that's actually happening tattoos are the least of your concerns in regards to professionalism.
If you must get something somewhere it will be visible, how this will be perceived will depend on the tattoo itself and on the field. If you want to be a teacher, doctor, or other sort of professional any visible markings might cause you trouble. Artists, on the other hand, can get away with basically not having anything aggressive and not having anything on the face or neck.
Generally speaking, I'd say get it somewhere you can easily cover, absolutely nothing on your face or neck, and nothing aggressive.