You get this as part of the tutorial. Just after your second race in the game you will be shown how to recruit new crew members. You will be guided through the process and earn your achievement, however if you're still curious, from the map screen, the recruitment tab is second from the left. This will take you to the Crew management tab. In the center is the Mechanic crew panel. Click on this and then an empty panel to view available mechanics for hire (if there currently are any) choose one and the achievement will pop shortly after.