How to check the oil?

I just bought my first car. My mother said that since I am 45 years old, it's time I learn to drive, and she said she is tired of driving me around, and when I turn 50, I must get my own apartment so I wont have her to drive me anymore. This angered me, but I bought a car just to please her. When I got the car at Smittys Used Cars, the salesmen told me to check the oil everytime I buy gas. I just bought $5 of gas and went to check the oil. I could not figure out how or where there's oil, or why there is oil. Then I could not figure out who to make the check out to. I asked the clerk in the PDQ where to find the oil and she told me it's in the back of the store. I found it, and there were black, yellow, red and green bottles. My car is red so I bought a red bottle since thats for red cars. I gave her the check for the oil so I guess I checked my oil. What puzzles me, is what to do with that bottle of oil? Usually I'd ask mother, but she is out with Bill. That's her new boyfriend. So what do I do with that bottle? I guess I am supposed to drink it but do I need to mix it with anything? One other thing. I had a nice policeman ask me where my license plates are. He said they should be on the car. I told him to call Smitty and gave him Smittys card. Then he wanted anothr license called a driver license. He gave me a piece of paper and told me to ask the judge. I am going to a cat show next weekend and there will be a judge there. I will ask him.