I'll do simple things like replacing a battery, changing the engine and passenger compartment air filters, etc. When I was in college and pretty much broke, I repaired a leaky gas tank, replaced my ignition wires, installed a car stereo, and did a few other things. I know how to change a tire, but I haven't had a flat in more than 30 years. I used to change my own oil, but it doesn't cost much more to let the shop do it, and they top off the fluids and do a mini-inspection at the same time, so it's worth it to let them do the job.