Year, make, model, mileage, relevant maintenance history (we don’t need to know the date of every oil change, but if you’re having problems with shifting and you just had Iffy Lube flush the transmission, that’s probably important), stored computer codes if any (Autozone will read these for free), and every detail no matter how small you think it is about the problem that you’re having (to eliminate the “My car won’t start. What’s wrong with it?” questions).