Every action depletes a small amount of stamina. Stamina recovers automatically after you play a game against another team, and increases more from every vehicle and property you own. When you select plays for your QB, if you are low on stamina, choose the most heavily trained play. This will be the one with the lowest stamina cost. Your stamina will not refill automatically over time. You will have to purchase energy drinks to refill your stamina, however at the end of the tutorial your income per game should be at least 15, which is enough to afford the cheapest energy drink. Balance your stamina by working on your plays, training, and social stats equally. Play in a game occasionally to earn money for more energy drinks.