http://www.carmax.com/enUS/car-search/used-cars.html
This is the CarMax website. My daughter is in one of there cars.
If you use Carmax you won't need to worry about what I've written after the Blue book link. The cars are good and there is no negotiation. The price marked is what you pay. The sales people are not on commission, they're on salary.

http://www.kbb.com/kbb/default.aspx
This is the Kelly Blue Book. If you like someting that you see write down, go to the site and print the pages with all the options. The go back to the dealership and check them off. The go back to the site and check out the value of a car with those options. It works even better if you have a lap top.

Caution:
Be especially leery of small lots that dealonly in used cars.

Tips:
If the front tires are good, look at the back tires. Is there even tread wear? If not there could be an alignment problem and they've swapped them. If the back tires have a good spot, bad spot,good spot, bad spot etc on the edge of the tire there is a shock absorber problem.
If the engine is clean as a babys bottom look at other cars on the lot. Are those engines also clean? If not, why did they wash that engine and not the others.
If you have a mechanic that you trust go and visit with him and tell him what you're doing and ask him about bringing three or four cars for him to look over for you.
Search the internet for pitfalls, stratigies and negotiating tips about buying used cars.
I don't know what state you're in but in Texas a use car is sold "as is" with no warranty and the seller is not required to disclose any defects.
You need to find out about your state. Do your homework before you go out. Decide what you can pay and don't go a penny higher.

DO NOT BE IN A HURRY!!!!!!

Psyche yourself up before you go in, to stand up, smile and say no thanks AND WALK OUT IF YOU DON'T LIKE THE DEAL. There is more than one car in town.

I'm not saying you need to treat the guy like the lowest of the low but always remember "You come first"
He's there to sell a car on commission and if you make an offer always remember that the next person to speak looses. Make the offer and shut up. Wait for him to speak the words yes or no.

Take someone with you, preferably an older man. All of your life you will find that because you are a woman the chances of being exploited or taken advantage of are greatly increased.

I hope you don't mind me sounding like an uncle.
Good luck.