To steal cars in Bitlife, you will need to find the option under crime in activities and click it. But to simply click steal cars is not enough, as you will also need to build a passion and ability to commit crimes starting at a young age.
The reason why is that things can go bad quickly for those not used to crime, so you will need to both create a character with decent athleticism skills and commit crimes every year as soon as crimes unlock for you.
You will also need to get your driver's license, as it's a requirement for stealings. That said, after you build up a rep, you should be able to get away with stealing cars - if something bad happens, you can always exit or restart Bitlife to reset the results, just don't age up.
Once you get a stolen car, you will be able to sell it by going into assets and clicking on the car, and then choose the option that says sell, and then pick a listing price for the car.
Eventually, after posting an ad for the car, someone will try to buy the car, but be careful as sometimes they may report you to the police. If this happens, just restart or exit the Bitlife app to reset the results.
Check out some of our other awesome Bitlife Guides!