Hi and welcome!
Mountain Biking is a great and fun sport you will get good physical exercise, nature experiences and discover places in the nature which you will never see on a normal bicycle ride. And perhaps a friend wants to join you it is a very good way to be social with other people.
Some day when you feel ready, signing up for a Mountain Bike Race will be a good challenge, learning to ride among many mountain bike riders, feeling the race atmosphere. But remember it is only for fun, so enjoy the ride.
Have fun. Henrik
To utilise your muscle power as much as possible and prevent exhaustion too quickly while mountain biking, it is important that you find a comfortable way to sit on your bike while riding