Well, I don't know a lot about cheap gas efficient cars sol in the States, since I'm European, but if I were you I wouldn't buy anything with over 100HP, this is your first car and considering your demands, I suppose you don't want to spend all your money on it, so buy something that'll last, and not too fast, so you won't destroy it after a few weeks, and more important, won't destroy yourself...
I might sound like a soccermom now, but I'm 18 myself and love cars, but I don't wanna spend every last cent I own on my car, so I bought a 1996 Ford Escort Wagon (a EUROPEAN Escort that is!) 1.8 Turbo Diesel, 90HP, drives decently and is fast enough to get me around in decent traffic + it has very good gas mileage.