#1 falls: Bail early. Don’t struggle to ‘save’ a ride that is going wrong. This will prevent most falls to the pavement and will leave you on your feet most of the time. In no case try to ‘save’ the uni. Let it go.
#2 tire pressure. It’s a personal thing. A harder tire is going to roll easier (and roll out from under you easier). A softer tire is going to take a little more effort to get moving but it also provides a bit of stability. If you find you are stalling out (pedals stop moving) then I would try a little higher pressure. If your trouble is the tire shooting out in front or behind you than maybe air down a bit. Don’t be afraid to experiment and see what feels best to you.
Even though you didn’t ask the same goes for seat height.
#3. Pavement vs Grass. Hard smooth surfaces are going to get you further much more quickly. But there are people who have learned on grass. You might consider safety gear to feel more relaxed while riding. It’s a good idea anyway.
Couple of other things.
Don’t be afraid to flail your arms around, a lot.
Every few attempts, take a moment and shake your arms around, make sure you are staying relaxed.
Most beginners pedal too slow. Don’t start going ‘balls out’ but work towards getting your pedal speed up as you start riding beyond one or two revs.
Keep your eyes up, or at least say 20’ out in front of you. The ground in front of your tire is not the right place to be looking. This will help with the balance on a uni.