How do I respawn my character after I die?

If you only want to go back to the start on death, that is very easy to do. Find the script that handles your car death and instead of destroying it or whatever, move it to the start and reset any variables (like health).

 vector3 startPosition = new vector3(0f,0f,0f); //Center of the world
 
 //Something happens to make the car die
 //...
 //...
 
 car.transform.position = startPosition;
 car.health = 100f;
 car.fuel = 250f;
 //reset any other variables here

If just moving the car to the start is not enough, you will have to show us some code that you have, so we can help you.