When you're stacking your dog, he should appear to be a square on a square on a square -body, head,muzzle. His forelegs should be straight down. rear legs kicked out. One thing you can work on right away is concentration. He should be totally tuned in to you. Also look at your dog from above; are his forelegs straight? feet pointed to the front? His topline should be level. Of course right now that's impossible, but when he straightens out it's something to strive for. If his topline is too high, kick his rear legs out more. Looking at the picture, he's A framed; his forelegs should be straight down.
The judge will be touching him - looking at his teeth & jaw, possibly pushing on his back end, raising his tail, etc. Exposing your puppy to as many strangers as possible touching him is a good idea, all the while you trying to keep his attention & holding his position.
If you're still having problems after classes, there are some little tricks that professional handlers use to help get them & keep them on a good stack. Nothing cruel; just a little boost.
Also keep in mind that puppy matches are more for experience than anything. They're great fun, watching these babies prancing & dancing. Showing dogs can be a lot of fun & you'll make a lot of friends.