You will need a fixture and a cord.

I don't know what kind of ballast you are talking about, so I will assume the usual 'horticultural ballast' sold by grow shops. You will need a fixture and a cordset. The cordset has a HID socket and a cord with the special end that will fit into the output of your ballast. If you want to do the vertical bulb thing where the bulb protrudes down trhough a hole in the top of your grow area, you're good with ballast,bulb,cordset.

Hope that helps, I really reccomend you talk to someone at a shop about this. It's not always obvious from the product descriptions that you need cordset, fixture, etc.