Here's what you need to do to become a medicine cat apprentice:
1) Have been a member on this site for a quarter moon (one week).
2) Have posted in at least one topic in the forest (not including your cat creation and Clan selection).
3) Have created a cat in the cat creation and gotten it approved.
4) Have selected a Clan and gotten it approved.
5) Have filled out the medicine cat apprentice template and gotten it approved.
When you have met all of these requirements, please pm your Clan leader and tell him/her that you have. They will hold your ceremony when they are ready.
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.