As a mechanic you have a few things to partake in.
Source Mechanical Supplies
Buy mechanical supplies at hardware stores to be able to work on vehicles.
Mod Cars at Mod Shops
At mod shops you can mod player owned cars to get money, every Mod Shop has a small fee on every transaction.
Drive to Clients
If someone needs a mechanic, ride up to them, offer your services and fix their car up. Exporters often need engine or tire repairs near the docks.