Each year they (MS xbox) run a program where you have to submit an detailed application on why you would like to be an ambassador.
I believe it is early each year they start accepting apps, with information on the xbox.com forums.
It is recommended that you have a good knowledge of the console (and how to address faults etc), have some activity on their official forums, be willing to speak to xbox live strangers confidently and without bias.
I will send you the info when the next opportunity comes about if you wish. Just be warned it is more work stacked onto your 'xbox life' so only apply if you feel you can handle the extra pressures.
- Ivy