A trainee is a player that has low age (perferably 18/19) that you are going to dedicate at least 3 seasons to training. Also, you want to get a trainee with at least allstar potental. I would wait until the start of next season to start training (about 8 weeks) becuase at the start of the season, young players are cheap becuase everyone drafts new players. When you trian, you want to get at least a level 4 trainee.
on your roster, you want to have 11 players. 10 of them can be good players, and the last 2 you only suit up for scrimmages and you want these players to have as low salary as possible. This is good because it is important to manage GS. GS can make a 50k salary player play like a 10k player. you dont want this. to ensure that all of your players have good GS, try to make all of your players (the 10 important ones) get around 70 minutes, and never more than 80min/ week.
For your rotation, I would get 3 big men (pf and C) that are capable starters in your league. (one doesnt have to be as good as the other, but the better the more your team will win. and 4 or 5 players that can play gaurd. What I do to make sure that my players dont get too many minutes per week, is I usually only start them once per week. Then the other game, I start them and have them play the full 48 minutes. (for training reasons)
Jope this helped!