You don't have to be on your own. Ask for rides and ask people to ride with you. At the registration desk tell them it's your first time and they will find someone to help you out.

Don't be shy! Ask questions! Specially the people in your own run group! As for gas it depends. I went to an autox once with a co driver. Have nearly 3/4 tank but we each did 10+ runs and by the end of the day the gas light came up. If you plan on doing 5-10 runs 1/2 tank should be enough. You never want full tank unless you are in the situation like I was.

When you walk the course, walk it with someone who is experienced that can explain the course to you.

As for number, they usually have paper number or a marker and paper to write your own number. Tape your paper number inside the window not outside. They fall off very easily.

Lastly understeer will kill you time (well so will oversteering), if you find your self understeering going through a corner or series of corners, slow down a bit more next time around.

Slow + smooth = FAST in autox.