And note that the training will only apply to a certain number of players, depending on what type you choose. Only players who played minutes at the selected positions will actually get training. Some other things to note about training:
For the most part people train single position (for example, passing for PG - only guys who played in PG spot get training) or two position (could be passing for PG/SG, or something like One on One, where you must do two position training). Two position training doesn't train as fast as single position, but something like 70% as fast. So it can be useful in the beginning to help build up a squad of good players.
Players need 48 or more minutes in the trained position to get the full effect. Fewer than 48 minutes and the training becomes very ineffective.
Most managers choose to focus either on guard training or big men training. Guards are typically a bit more expensive to buy, so many people recommend training guards and going on the transfer list to buy some big men. But it may depend on what players you were given when you started.
Training speed is based on age, height, and the values of linked skills. Note it's not affected by potential. It can take a long time training before you bump up against the potential cap, so don't be afraid of players with starter, star, or allstar potential. Similarly, don't overpay for old guys with high potential that they're never be able to reach.