First, don't drive it now. Go get couple quarts of oil

in any parts store, walmart or even corner stores (might be pricier though). Add oil to correct level before starting your engine. Not sure what kind of car you have, but generally 10w30 for gas engine will do.

Crawl under your car to see where the leak takes place. if it's because of loosen nuts, tighten it. Cautious on safety.

Never, never go to the same store. If you want, you can write up complaints on websites so to let other people know. Arguing with them will yield nothing.