Ok, now you have players in your hand. It's time to give them some training. You can skip training, but training players will give your team advantage over teams which consist of raw, untrained players.
Attack training will improve your players attacking skills; similarly defense training will improve your players' defense skills. Fitness training is important in order to make your players fit and not making them out of form very often. Remember, every training is expensive and cost you credits.
My team -> in this page you can view complete stats about your players. You can see player type, player attacking and defense skills, fitness, stamina and player form. It also displays the card(s) that player is holding, if any. It also shows whether he's injured. If a player is injured he can't play next game. Same goes for player with red card and player with two yellow cards.
Most important setting on this page is, you should decide which player should play in which position. i.e., you should decide who will be your attacker, which players will play in your midfield and who all will be defending your post. While choosing their position look into the player type. Though you can assign an attacking role to defender, it's not wise to do so, coz it will affect overall attacking value of the team. Same goes for every player. Try to choose the player fit for that position. Also make sure a goalkeeper is selected and assigned to the appropriate slot.
If a player holds a yellow card it's always advisable to make substitution for him, else chances are high he will get another yellow card or red.
Give proper substitution to your players. If you decide to make them play continuously, it will affect their fitness and eventually their form. If you decide to make them rest continuously it will affect their form. Best results can be achieved by maintaining a rotating routine.