Couple of things since you are new to car rentals (I rent them frequently)
1. Are you at least 25 years old? Most companies won't rent to people under (some exceptions at a MUCH higher rate)
2. Make sure you rent a car that has unlimited miles. Most do, but be careful
3. Make sure you are returning the car to the same location you pick up from. A different location for drop-off can be very expensive
4. Understand it is doubtful you will get the exact type of car you booked. Car companies rent by "class" not make. If you see a RAV-4 in the booking description, it could mean a a Ford Escape, Hyundai Santa Fe, Dodge Journey or a RAV-4.
5. Understand not only the amount of seats, but also the ability to haul luggage. This is KEY. If you have a party of 7 adults, you could rent a Chevy Tahoe that seats 7 adults...HOWEVER, the 3rd row required for the last 2 seats when in use as seats takes up nearly all of the cargo area. You literally could not bring any luggage in this scenario.
6. As stated above, make sure you understand the insurance requirements. Most of the time your own personal car insurance will cover you, but you need to verify.
7. Understand the fuel options. I never pre-pay for gas, it is very expensive and I never know how much I am going to use on a trip. Skip this option and buy gas at a station before returning
8. Make multiple reservations. It does not cost (at most places) to make a reservation. I always have a second reservation in case something goes wrong with the first reservation. This has saved my bacon multiple times. Remember, there is no law that states that just because you have a reservation that they have to save a car for you. This can be a very bad situation on busy times during the year.