Q: Zenon bulb?

Zenon bulb went out on my 2003 Nissan Maxima. It will cost approx $300 to have dealership replace...I was looking to buy the bulb and just replace myself to save $. 1 - how do I know which bulb is the one I need? There are many out there... and 2- anyone have the instructions for how to replace? Thanks for any help!