I know quite a large amount about cars, so if you have any questions just ask.
Most importantly, know what you want to buy before you go. Make, model, options. Search online for your car, find the best deal. Do a carfax on the car. If the price is great and the carfax clean, then move on to knowing what to look for.
And honestly, if you know you don't know what to look for, take it to a shop that you can trust. They'll do a pre-purchase inspection for 50-100$ and let you know if you should buy the car. Doing so can save you thousands later.