cluu1, get a small pocket tape measure and carry it with you when you go to try out bikes. You should know within 1/2" what your seat height adjustment should be. Measure from the center of the bottom bracket to the top of your seat along the length of the seat tube.
You don't want to go testing bikes with whatever position they've got the seat at. As you're comparing bikes go ahead and measure the seat and handlebar height (from the ground). This will give you something to remember when comparing bikes. If you try one bike out and the handlebars are 4" above the seat it's going to ride a lot differently than another bike that has bars 2" below the seat. After that measure the seat to bar distance, you can do that with the tape measure or your forearm to fingertip as a rough guide.

I bought my first bike at age 17 and took it on a 40day tour after highschool. The shop took me through the basics of fitting.
That's why I'm recomending you check out the bike shops offerings given your initial post that you're not sure what to get and your willingness to buy a road racing bike when you stated a preference for a hybrid. Unfortunately you won't find any new drop bar bikes for $500 but you should at least get the basics of seat/bar position established before you let a Craigslist purchase pick that out for you.