Thanks Perpete and Big Njord. I've read everything in the link Njord gave me and I believe I have a fairly good understanding of the situation now. And from what I understand, I've come to this conclusion:
The best strategy for training is to pick a position, find 3 young prospects, train them, sell them for huge profit to buy trained players for the team.
1. For example, I think I have 1 player that have good potential as a PF. I should buy 2 more.
2. I should only focus on training these 3 players. The rest of the team won't have any training except stamina and Free Throw training.
3. Sell all other players that won't make it to the team and I can't train.
4. Seasons later, when those 3 are ready, sell them and use the profit to repeat the process and buy good team players.
How does that sound?
I don't want to buy players at the moment except for positions to train.