First let me apologize for the over-zealous responses of my fellow unicyclists. They sometimes lose sight of the fact that people want to just learn how to ride a unicycle and not specialize in one area with expensive custom equipment. You don’t need a trials unicycle or a MUni or a unicycle with all kinds of reinforcement or high-tech parts. Unicycles are strong if you’re just riding along and falling off while learning. You may not even like unicycling so why sink a bunch of cash into a Ferrari or a tractor when you just need a Ford Escort.
The 24" United trainer (now I think just called the Trainer) is fine for learning. Something similar to it is also fine for learning. Any of the Torkers or Suns or Zephyrs or variants are OK. The only musts are a pneumatic tire and cotterless cranks. Main cap bearings are a plus but lollipop bearings are even OK for a learner. Those single wall steel rims are fine for a first unicycle and strong enough to hold your weight. My brother in law was over 300 pounds when he picked up a 24" United to get some exercise. That unicycle is still with him.