When you start the game you are given a default lineup of players for your team and as you progress the game lets you recruit real FIFPro players to build your own team. You can also choose the formation you want your team to play in as you take on any team who stands in your way as you rise through 6 Leagues (Academy Division, Division 3, Division 2, Division 1, Junior Elite Division and the Elite Division) on your way to the top of the prestigious Elite Division.
For your players to perform to the best of their ability it is important that they are in their correct positions, you can easily tell if they are or not by the colour of their icon. If the icon is green your player is in their correct position, if the icon however is red you should consider moving them or changing the formation of your team to get maximum efficiency.
To help make your team as green as possible you can always do trades or search the Transfer Market for new players, there is also the option of upgrading players you already have on your team in the Player Development section for 50 coins a session which although expensive means you will not have to transfer in many other players.