Find someone else to go with or tag along on a group run. After a while you'll find out what you need just by watching everyone else. Some stuff will be specific for the type of terrain you go on, whether it's rocks, snow, sand, etc,. Other items will depend on the type of trips you do, like day trips, multi-day or weeklong. But there are some items that are pretty universal
Some of the basic off-road (or even on-road) items would be
Air compressor - most come with a pressure gauge, but if not carry one. A cheap one will work, but if you plan on doing this long term a good compressor is worth the investment
Tow strap - or kinetic strap, but I've only used a tow strap and never really felt the need for anything else
Tire plug kit
Basic tools (depending on your capability) - The minimum would be some screwdrivers, wrenches, pliers
First aid kit (again according to your capability)
As you get more advanced you observe what everyone else is using and that will give you a good idea of what you need. A lot of the stuff you'll see isn't necessary, but does make life more convenient and the trip more enjoyable.