AMA Arenacross questions

The biggest problem for most new riders is the whoops. They are the most difficult. I would recommend getting someone to build you some to practice.
Track walk is important.
Most of all, AX is a little more stressful than a normal race. Something about everybody being packed in to small pits. So be early and be organized with all your stuff, and as little stuff as possible.
Also, wear lots of gear. AX tracks are packed so hard they have the firmness of pavement. At least the ones I raced in Baltimore, Hampton and Youngstown.

You can disregard the comments about the whoops if you are talking about the Expert class on amateur days, I am assuming you are meaning the Lites Pro class on Fri/Sat.

As Moto Soul said above, get comfortable with big whoops. Here and there they have a few rounds without big ones, but most of the time they are big, round, and far apart. If you can't get through them, you'll lose a ton of time and lose your flow every lap. Keep in mind, the whoops are usually 1/4 of the track in Arenacross.

Get a set of suspension setup for AX/SX. It doesn't have to be A kit stuff by any means, but get some stuff sprung and valved for the indoors. This will help you a ton in the whoops as well.

I always recommend new riders build a set of 4 whoops to start, get those down, then add a 5th, then a 6th, etc. This will help you get the confidence up to come in fast without having to worry about committing to a whole section without dropping in right away.

You'll usually go a tooth or three larger on the rear for AX. It's 2nd/3rd gear with 4th or 5th in the whoops usually.

Jumps aren't usually extremely difficult for your average expert rider, but getting comfortable getting setup with a steep face immediately out of a super tight bowl turn does take some getting used to.

From there, practice sprints. The tone of the night for AX is set during timed qualifying. Run a good lap, get a good gate pick for your heat, get a good start, qualify out of heat well, good gate pick for the main yadda yadda. Reverse that, and you are in for a long night. Races go fast and there isn't a ton of room to pass (again, see comments about whoops).

Other than that, the only other thing is getting used to riding in heavy traffic. The only real way to practice that is experience, but you can get something similar to that on amateur days as well.

Try and remember you're doing it for fun and for the experience the first few events. That should help you can keep the nerves down, you'll be able to stay focused and keep the day "slowed down" a bit. Otherwise, it'll fly by and you'll have never even had a chance to settle in and run a laptime and you'll be in the stands for the night show.

I believe there are also some indoor race series near Tacoma and Olympia. Those would help you get comfortable in the tighter confines.
I've never raced arenacross but I've raced indoors on tamer tracks quite a bit. You'll probably want to slide your forks up in the clamps for easier cornering and possibly run less race sag than outdoors.

I would avoid moving the forks in the clamps. What you gain in ease to turn the bars you lose in both stability, traction, and more importantly length of the bike which will drastically help in the whoops.

Keep the chain adjusted back as far as you can as well.

And what you said about riding indoor places or even more local events first is absolutely a great idea.