Additional cars will become available through natural progression as you win more races. Usually by level 15, you should have three cars to choose from. Collect from the premium crates that are given free every two days and the regular crates. Win stages to have a chance of winning car cards. Tap any car desired and complete the stages or requirements that are listed. You can also spend in-game currency on the Black Market to get blueprints. Complete a Special Event to get a reward that may be a new free car. When your car collides with other cars or obstacles, it will become damaged. However, besides losing some time and speed, the damage will not be permanent. You will not have to repair your car or spend money to get it back on track.
Experience
The Car Series races will reward more experience than the Underground varieties. Win them for as long as possible. Your early goal is to unlock everything as soon as possible. Complete all the Car Series, refill your energy and take advantage of the free first time bonus, then participate in the Underground races again. Try to get to level 15 quickly.
Jumps
Stay in your lane when you have jumping boards on the track, as they will always be on each lane.
Nitro
You can win races and events using a less powerful vehicle against better competitors by using the Nitro appropriately. Draft, drift, perform jumps, go though Nitro Boost (the blue arrows), and attempt near misses to increase your Nitro. Performing those actions also increase the amount of money awarded after the race is completed. You can use Nitro at any point in the race. Do this as often as possible, as soon as it is ready. The best method to fill it is to drift, however be careful when doing this on sandy track surfaces.
Upgrades
Take note of stages that you can complete and replay that offer a guaranteed reward that is required for upgrading your vehicles. You can go to the garage and tap a desired item to view all the stages that you can replay to obtain it. The more upgrades available, the better your car will be, which will allow you to win races and unlock new stages easier. Early in the game, choose a car to focus on and use it to progress the story going and win races. Because upgrade parts can be used on any car, begin stockpiling them for use when you get a very really powerful car. Quickly upgrade it after it is obtained.