There are three main methods to leveling up your character in Madden Mobile. The first is by doing certain tasks in-game, such as completing a set number of games or getting certain achievements during gameplay.
You can also go into the team management screen and purchase a rookie pack, which will give you enough experience to level up one time. The third and final way is by sending players to the Hall of Fame. If you send certain players on your team to the hall of fame, they can help generate XP for other teammates as well.