The idiom 'to get your head in the game' means to pay attention to what is going on or to focus on a task or goal without being distracted. You might tell someone to 'get your head in the game' if they are not performing well or being unfocused or to remind them to keep their mind on the task at hand.

1. This task is too important for us to waste time or make silly errors, so I need you all to get your head in the game!
2. Brenda seems very distracted. If she doesn't find a way of getting her head in the game she won't get her article finished before the deadline.
3. With the holidays coming up, I'm finding it hard to keep my head in the game at work. I need to refocus. 

1. You are not playing well today. You need to focus and get your head back in the game.
2. I haven't been on track with my writing project I need to get my head back in the game.