It is not something to explain in few sentences. It depends on the opponent, the tactics, your training. etc. You will have to learn a lot.
Saying that, if you want to know what to do while studying the game, here are some simple tips. I am new in the game as well, but I am doing very well so far following these tips
1. Look for the players with the highest salaries on your squad. they are probably your best players.This is a generalization, that not always is true, but the earlier in the season, the better chances it is true.
2. Be fIexsible with the role definition, especially if your squad is not balanced, for instance, if your second best small forward is much better than your best shooting guard, you should consider using one of the small forwards as a shooting guard.
3. A young player with a high potential is the best player to train. If you have such a player, consider including him in your oppening five, even if he is not the best player for that position, and define training accordingly.
4. Define your tactics to benefit from your best players. If your Center is the best player, it might be wise to play low post or look inside, and vice versa
I hope it helps to start the game.