Just browsing your roster, you don't really have anyone you should be training. Try looking on the transfer for an 18 yr with at least allstar potential for under 50k. You might also want to consider training a guard or a post as your first trainee. Training SFs is difficult because of all of the out of position training you have to do.
Keep in mind that short players train guard skills quicker than big players and post skills quicker than small players.