The only way to get prepared is to get on and start working it. There are loads of threads in this forum that go over the learning process from many different points of view, for people of various ages. Advice we would offer would differ between a teenager, a grown-up like yourself, or a small child. For a guy your age we would assume you want to be more methodical than a teenager, and that you won’t bounce back from infinite falls like either of those other two demographics. 
Tire pressure should be low for your early lessons. Higher pressure makes the wheel too easy to twist from side to side; you want rotational grip. Try it around the 45 psi for now. (Note: “Correct” tire pressure is dependent upon rider weight, tire width, riding surface and intended riding style, among other things. But beginner unis all tend to have wheels in the 1.75 - 2" width range, so it’s easier to give out meaningful numbers.)
Many of us bought safety gear after finding out it would be good to have. I learned to ride when I was 17, and bought my first safety gear at 18. This was from my attempts at practicing for Track racing; always falling on hands and knees! I am now a few years older than you, and think you would benefit from wrist protection (protecting the bones more than the skin), followed by knee and shin protection. Start with something cheap and see how it goes.
You’re going to sweat a lot, at least if you have to practice outside. If indoors, stay away from stuff you care about, or anything made out of glass.
Also note that I once punched a hole in an interior wall – while trying to demonstrate to someone else how the uni can shoot out and hit the wall, which can be painful in the crotch area!
If you’re outside, I guess sweat is a given. I’ve been to that part of the world once, doing shows in Singapore for a couple of weeks at Clark Quay. It was great fun, but boy did we sweat buckets! 
To reduce “empty” time when learning, concentrate on the riding/balancing part. Work on making half-revolutions, regaining your balance in between while holding onto your support. Getting back on takes up a lot of time, so keep the distances short; learn to stop and get balanced before going again. Make sure you spend more time on the unicycle than on these forums. 