As stated, you need a ballast. You will also have to decide what wattage you want and what "flavor" of light you want to use because they are not interchangeable. A 400MH ballast runs a 400MH lamp...a 400hps runs a 400 hps lamp....etc.
A conversion lamp allows the user to switch from Halide to hps..or vice versa, using the SAME ballast by merely screwin in a new *conversion* lamp.
good luck
bt dt