I did unicycle a long time ago. I can definitely recommend it. It's a lot of fun. I never tried mountain unicycling though.
As a start I would get a simple unicycle, curved saddle (no bicycle saddle), with plastic or metal reinforcement in the front and back of the upholstery. You'll be grateful for that. Look that you can easily hold the saddle up from below, so that you can easily hold it while mounting.
Aim for a small wheel, so that you can step on and off easily.
Get pads: Knee, elbow, wrist/hand, helmet. Just in case
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Start training on tarmac with a rail beside you. Bridges without traffic work quite well. Just hold yourself on the rail, one foot on the pedal, get up into the saddle and place the second foot and try to ride along the rail and you'll get there eventually
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Good luck and enjoy!