I get about half a rev and then stall. What is the trick to not stalling at 6 and 12 o clock?
momentum - tell yourself to keep pedaling.
I’m no great unicyclist myself, but for me it just came down to practice practice and practice.
I know a lot of people recommend the wall method, but I learnt to go by doing a supported mount, looking across to the corner of a room, leaning forward and just pedalling like mad. Come off after half a revoluntion over and over and over again, then maybe manage 3 revolutions, but before I knew it I was crashing into the wall on the other side of the room 
And yes, momentum, that’s where the pedalling like mad comes into it. It’s tempting at first to take it steady but faster is easier when you’re starting out.
I had this problem in two different situations: when I first started, and when I started riding up hills. For starting at first, I found that pointing my toe down at the bottom of the stroke helped me get over the 12-6 stall:
It took a few hours to get past this stall, but as soon as I got past the first half revolution, I could ride a few hundred meters.
The second time I had this problem was when I started riding up slightly steep hills. I’d push too hard on the pedal and end up jumping off the uni when my first foot hit the 6 o’ clock position. It took several hours of practice on several days riding the same little hill to get myself to stop putting pressure on the pedal at the bottom of the stroke. I can now make myself stall in the 9-3 position if I need to, instead of getting stuck at 12-6. I think this has an official name, but I don’t remember it…
The problem may be that you continue to push the pedal downward even when it is at its lowest position. Because of that downward force, the pedal cannot come up. So try to stop pushing the pedal downward when it is almost at the bottom, then let your momentum carry you past the dead point (6-12 position where you have little control), then start pushing down on the other pedal as it starts to come down. Don’t go too slow or this becomes more difficult, because the time spent around 6-12 becomes relatively long. On a 20" unicycle, walking speed is about the lower limit for a beginner to be stable.
Note that the “dead point” isn’t really dead. You can push the bottom pedal backwards, and the top pedal forwards, relying on a combination of friction (between sole and pedal), and angling your feet appropriately. But I’m not sure if you should try to use that effect as a beginner. Using momentum to get past 6-12 is probably easier.
Think light and let your feet go in circles.
You are probably putting too much pressure on the pedals and not relieving the pressure when you get to the bottom of your stroke like Klaas says.
Try concentrating on getting more of your weight on your seat and less on the pedals and see what happens. Raising your seat might help for this.
+1 on all of the above
Also in addition to pointing the toe of the bottom foot, pull the toes up allowing the heel to come down on the top foot. This would still give you a surface to pull back on (bottom foot) and something to push forward on (top foot), allowing you to alway be applying some pressure on the pedals propelling you forwards.
On the circles, it helped me to remember the feeling of pedaling my bike w/ clipless pedals and how if I wanted to maximize my efficiency for spinning fast or climbing a hill, I’d make the circles as smooth as possible. Also as I implied above I’d be always applying some pressure all the way around, but on a uni w/ platform pedals, you can’t pull up of coiurse.