George is spot on. It is all about what is in front of you. Also know the equipment inside and out. You need to know how the GPS works and what to do when it doesn't, how to fix the normal issues (Tires, Belts, Axles, Hubs), know how to communicate to your driver, how to communicate to race support and how to communicate to your pits. One point on communicating to your driver. If you are not paying attention, he will start trying to figure out what is going on and lose focus. Figure out what bothers your driver and give him updates. I worry about belt temp and Faster/Larger Cars, so my Co-Dawg gives me updates even when no issue.
Also know what to do when you crash, because you will. Get help on the way and practice getting you and your driver out of the car quickly. Also you have to be the one that is calm and cool and be the one to jump into action. There is an issue, YOU fix it. You can ask the driver to come help if it is too big of a task, but you are the guy.
Now for a quick story, I have not been racing very long, but I have given my co-driver a concussion and I have knocked him out cold during a race. In San Felipe we hit a tree, he hit is head the A pilar and at the 500 I hit a whoop so hard his head whipped forward and he went out. You are going to take a beating. Make sure you are both focused on your agreed upon priorities. Our number 1 priority is getting home safe, number 2 is winning. Different teams have different priorities, make sure you and your driver are aligned.
Good luck...