It´s pretty easy actually. In Life Mode you pick a car to begin with (there´s 12 cars available at the start of the game), and go racing. You have a few races to select from, with different difficulty levels and oponents.
Each race award you points, which upgrade your driver´s and car´s level. Every now and then you are awarded with a car that you´ve raced against, and it will available for you to pick on you car list. You don´t need to buy any cars, you can keep changing between the cars on your car list anytime.
Hit Points (HP) works like a life gauge of your pilot. Each time you go off the track, hit the wall or an opponent you´ll lose some HP. If you run out of it you won´t be able to race on the next week, so you´ll have to either change cars or rest.
But each action you take, being racing, changin cars or resting (when you HP is empty or low) takes a week time to complete, and your goal is to be the number one driver on the rank against other 1000 pilots within a limited number of weeks, so you gotta plan your actions.
Are you sure you´re playing in Life Mode? The only reason for you to be repeating races is not winning them I think.
And try to play the Driving Revolution tests before going into Life Mode, it´s pretty fun and you´ll learn the game´s physics.
I hope that helps, cheers.