When the Admin/Guides think that you are specifically trained to do it, e.g. a good helper or member, they will pick you to be a Guide or Co-Admin, but it usually takes a while for them to choose.
Lots of ppl ask about being a guide/moderator. There's a topic pinned in the User's Guide to TamaTalk forum called "How to be a Guide" by Bell Sprout. The best info/advice I can give you is to read it carefully and try to follow the suggestions over the next few months