This applies to all races and is also how I go about the game as I know a bit about cars, but not enough...
1) Go in to beginner/amateur/professional/whatever and then select the event that you wish to compete in.
2) Once on the screen where you select the track there will be an option to view typical competitors in the race, they are listed with the most powerful at the top. Make a mental note of which car is the most powerful and it's power.
3) Go back to the track selection screen again and select your garage in the bottom right
4) Only the cars that are valid to enter the race will be listed on your 'premium' and 'standard' tabs, see if you have a more powerful car than those listed as typical competitors, if you have just use that.
4a) If you have a car valid for the race but not powerful enough, exit out and tune that car up so it's more powerful than the other cars (This saves you spending money on a new car if you already have one that is valid for that race).
4b) If you do not have a car valid for the race, find the most powerful car you can in the dealership/used cars from those that were listed as typical competitors, then tune it so that it is more powerful than the others
5) Enter race and win