I don't know much about cars either, but after looking and comparing prices of cars you see for sale you can get a fair idea of what particular models are worth. It took me about 5 weeks to actually buy my latest car after I started looking, but that was because I wanted to know what I was getting myself into, and what I should be looking for, whats good value, ect.
If you know what model your after (eg, 1990 Camry), then just look up on all the car yard sites to get a good average price, and look at private sales to see how much it varies, ect.