I guess it has been too long. 
You don't use a transistor to operate a standard R/C servo from an arduino. You just wire an arduino output pin to the servo's control wire, a ground wire from the servo ground to an arduino ground pin. Then you need to supply a +5 vdc voltage source to the servo's power wire. If it is a small enough servo and you don't have a large mechanical load applied to the servo you MIGHT be able to power the servo from the arduino 5V pin, but as current is somewhat limited at that pin the usual recommendation is to use an independent +5vdc power supply to the servo power wire, and the ground of this external supply has to wire to a arduino ground pin.