First, mince some onions (you're supposed to use shallots), and ginger, more ginger if you want it more spicy. Then mince some ground pork. I also throw minced garlic and pepper into the equation. Mix it up in a bowl, and add soy sauce, the amount depending on your salt tolerance. It definitely shouldn't be runny.

If you want to fry potstickers in vegetable oil, often with leftover wontons because they stay moist longer, seal the meat tightly. Have a little water bowl beside your plate and wet the edges of the wontons with your finger where you want them to stick. It's probably easiest to use chopsticks to put the filling inside, because you only use one hand. If you're doing wonton soup, don't make an impermeable seal, because you want some of the filling to escape into the broth. If the meat isn't tender and only recently not-pink, you're cooking the wontons too long; it should only take a few minutes. When I'm feeling cheap, or running low on meat I put in a tiny bit of minced cabbage filling too, and it still tastes great. I'm trying to eat fewer intelligent animals, so I've also filled wontons with minced cod or clams (minus their stomachs), and they still taste like wontons. Also, I've used normal white and yellow onions too, though this is apparently very not kosher. It tastes great to me.

Also, you can freeze the wrappers for a little while if they're tightly wrapped in plastic. And freezing ground pork isn't such a disaster either, so you can always have the mixings for wontons in your freezer.

You'll be amazed at how cheap, easy and tasty chinese appetizers are. I eat potstickers on rice all the time, and chop them up a little to flavor the rice. Potstickers are awesome because you can make a big batch with rice and eat them for lunch the next day.