Here's what you need to do to become a Deputy:
1) Have become a Warrior
2) Have achieved the rank of experienced or senior warrior
3) Have mentored at least one apprentice
When you have met all of these requirements, please pm your Clan leader and tell him/her that you have and want to become a deputy. They will hold your ceremony when they are ready. Also, the deputy slot must be open in order for this to happen.
But, not only do you need to have met the requirements to become a deputy, but you need to be seen fit in the eyes of the other warriors that you deserve to be one.
P.S. At the moment, I have taken over all leader and some apprentice training duties. So please pm me if you have met the requirements. Read this for more information.