Which Motor to use????

Hello. I am making a Segway-like thing and am wondering what voltage of motor ill need. this will be a sit-down type

I assume that you must tell more requirements about what you expect the thing to do to get a reasonable answer. - what speed - what acceleration - what load - what distance/range should it be able to do

Indeed, sizing a motor is what you do after working out the mechanical requirements, not before. Work out the mechanical arrangement (any gears/belts etc) and then work out minimum torque and speed needed of the motor.

This has been done before. Do a bit of Googling and see what other people used.