There is no one definitive answer to this question. Different ways of getting cars will work for different people, depending on their gameplay style and preferences. In general, however, you can purchase cars from the car dealership or trade in your old cars.
The Sims 4 comes with a built-in mod manager, which you can use to add mods to your game. To add mods, launch the mod manager and select the sim from the list of sims that it will open in. Click on the Add Mod button and follow the instructions on the screen.