When you actually get the car rented and go to pick it up, a little more advice:
They almost always try "up-selling" at the counter. Bigger car, more insurance, tank full of gas, GPS, ?? Hold your ground and don't say yes to anything but what you've originally decided. Returning the car with the gas tank full is (IMHO) the best option. It is nearly impossible to return a car with an empty tank, so with that option you mostly end up giving them a bunch of gas.
When you go out to the lot to actually get the car, take time to walk all around it and inspect it for damage. Have them note any scratches, dings, etc. before you drive away. Some people even take pictures of any previous damage, just to have proof they didn't do it.
Take note of which side you fill gas before you get into the car.
And when you get in the car to drive away, take time to set the mirrors and seat to your liking, and figure out how all the buttons and knobs you will be using work. You don't want to be hunting down the headlights for the first time when screaming along a freeway.
Maybe even drive one loop around the rental area before going out on the major streets - just to get a feel for the car.
Have you looked at Hotwire and Priceline rates yet? Availability for under-25 drivers varies by location, so I don't know if either would work for your destinations. While you won't know the rental company until you complete the deal, you always get one of the majors.
Romelle