Some cars are more reliable than others, so the whole mileage vs age point is debatable.
With that said, once you have something you're considering buying, have an offsite mechanic inspect the vehicle, and also get a
carfax report on the vehicle. Also, take a parent or adult with you when looking at cars, and try to take a test drive for at least 10 miles (cover highway, and rural roads).
I certainly would look at the KBB and
Edmunds.com pricings for the vehicle. They'll give you a ballpark figure of a fair price for the car, but they're not exact. Condition of the vehicle, mileage, popularity and appearance will all effect the price.
Edmunds is also a great place to look at various ratings for the car you're looking at.
If you're going to be getting a loan for the vehicle, shop around for good rates first, and then see what banks will approve you for. This'll let you know what you're able to spend, and will give you a good bargaining point with the seller. If you know you're able to get a loan for 6k, and they're selling the car for 5k, you can try to get the price down by saying that you've already got the money, and you're ready to take the car that day.
If you've got cash, that's even better, and gives you even more power.
Also, and I love doing this one, when you're ready to buy it and are sitting down with the seller doing the paperwork, ask them to cover tax and tags. It's a couple hundred bucks, and it's better it comes out of their pocket than yours. At this point in the transaction they've already started some lengthy paperwork and they're happy they've sold the car, so they usually don't flinch at this request.
Don't ever act excited or like you
need a car, or tell them how much you have to spend. Those are all things that will give them the upper hand when it comes to negotiating price, and you should
always negotiate the price.
Good luck. Buying your first car (if this is) is fun, so long as you don't get screwed.