I need help deciding who to train...
For starters take a read of these articles:
As a general rule you should start by training players who are under the age of 20. Mediocre is about the minimum you should start with (the higher the skill the better).
*After reading the articles, take a look over the older training threads on the Newbie Board (click view older threads). If you still have some questions then open up a new thread on the Newbies board.