When learning to stall the coping in a halfpipe, it's usually best to start of with the basics, and definately start off in a small mini ramp.

Backside Stall
Start off by pumping the ramp until you can comfortably reach the coping every time. Once you have done that try to tap the coping with your skates, and get higher so that you could feel yourself going for a backside stall. When you feel you're getting good enough, go for it and land in a backside stall on the coping. It's best to go for a backside stall first, as once you have done it you don't have to worry about trying to spin round to land in the ramp again. When you approach the other side, try the same and keep going until it's consistent. Once you've got it good. Get on the deck of the ramp and drop in and do it straight off. You should have it on the lockdown by now!

Other Stalls (Soul, Mizou)
Now you have the backside comfortably, you can work on more stalls. When going up to do a soul or mizou etc, go at it like you would stall a ledge. Do a small jump up and aim and land on the coping in the stall, you may find this method easier than trying to go up and smoothly lock on straight away.
 Once you have got this, try going up and putting your feet into the position, without the jump. This not only looks smoother, but you'll find it easier when it comes to grinding as you will find carrying your speed through from the ramp to the grind much easier.

Spins in and out of stalls
When you want to start spinning into and out of stalls, as always practise the spins on a ledge first, and get comfortable at spinning into and out of the stall. Then take it to the ramp and land it!

Hope this helps and good luck !