So, in order to progress through career mode, you're going to have to earn or buy more cars and use them, in turn, to earn cash in the highest-level races available to you. The earliest races will usually offer you around 1,000 credits per race, which will be enough to unlock new races as you increase your level; you'll also increase your stable of cars as you win events, allowing you to try the different races as you unlock them.

Now, depending on your skill level, you may either want to find races that provide an even match for your cars, or, if you're just looking to progress a bit, you'll want to find races that you can easily win with a certain car. In general, the quickest way to cheaply earn cash will be to hop into your most powerful car and head to the races. You can repeat races for more cash as many times as you like, so if you find a high-paying event that you can win easily, feel free to start racing it over and over again, if you like, to build up more cash for upgrades and new car purchases, and to earn more levels to unlock new races and earn new free cars.

Keep in mind that the gap between levels is going to increase as you go ever-higher, which means that the early, low-paying races will eventually stop being an efficient way of increasing your level, so you may want to find a high-paying event and see if one of your cars is competitive in it, and if not, then either buy a new car or upgrade one of your earlier ones to give you access to more cash rewards. You increase your level by earning more cash, not from your total cash reserves, so there's no reason not to invest in some upgrades if you think it'll help you win races that wind up paying more.