Adjusting air pressure in the Tyre /Tire
I assume that:


you run stock Trek 3500 so you have 2 inch tyres
you are not 200lbs (judging by your photo, location and fever for fitness in your profile)


Start with 40 PSI on the rear and 35 PSI on the front and ride at that pressure for a couple of rides to understand how it feels. Then adjust +-3 PSI to see how the handling changes. If you ever get a pinch flat increase 1 PSI on front and 4 on the rear.

For trails, aim for a 10-20% height reduction on the tyre height on where it touches the ground when you are standing on the bike.