You should do this right away. Decide if you want to train big men or guards. If big men then get a couple of 7'5" 18 year olds even if they have low potential like 6th Man. But probably no lower. They shouldn't be very expensive. If you're going to train guards, get a couple of 18 year olds who are no taller than 6'2" (the shorter the better) and train them.
Potential is not very important right now, just don't waste your training minutes on older players or bad players. Oh, and for these 18 year olds, you should make sure that they have at least a starting salary of $3,500 or so. Obviously the higher the better, but they tend to get very expensive. If you can't find 18 year olds, 19 year olds are ok substitutes for now.