If you want to become a mechanic in Bitlife, you will first need to create a character and then finish high school. After this, you can become a mechanic by finding the job apprentice auto mechanic and applying to it.
However, you may run into a problem with finding the job. This is because what jobs spawn in Bitlife is random, so you may need to check a few times before one will show up. To do this, simply age once and then check the job list again for the job. If you don't see it still, try restarting the game and then check again.
Related: How to make friends in Bitlife
Once you do find it, applying to it is easy, and you should get accepted and hired with no problems. Becoming a mechanic will also earn you around 25k a year and even more if you get promoted. Because of that, you might want to figure out the best ways or how to get promoted in Bitlife.
In short, if you want to get promoted just work hard and put in a lot of hours, and you should increase that 25k to a bit more each year. It also helps if you don't annoy your supervisor as they will take it upon themselves to ruin your life if you give them a reason. That said, do all of that, and you won't have a problem getting promoted as a Mechanic.
For more on Bitlife, check out How to develop an Addiction in Bitlife and How to complete the Motorhead Challenge in Bitlife.