First, this is a simple thing that many people do very differently, but it all ends up pretty similar. Also like a sandwich, the fillings are the interesting and different things that you change and make up and try things!
SO! First cook yourself some short-grain rice. You could use other rices, but the stickyness of short-grain makes rice balls easier to hold together, and I like the flavor of short-grain more than others.
You now have two choices: salt water for your hands or salt the rice directly to taste. If you are going to roll your onigiri in furikake, or prefer a less-salty rice-ball, I would suggest making a big bowl of salt water. If you want simple fillings, (like cream cheese or sausage bits) salt the rice.
NEXT! Make/assemble whatever you’re going to put on the inside. In general, things with a stronger flavor are nice because you’ll be eating it with a mouthful of rice, and nothing too wet or it’ll make your onigiri fall apart. Kiki loves Neufchatel or other cream cheese fillings, I like a piece of sausage, and my wife loves a oil and vinegar tuna salad. Also a big fan of the fishy furikake, nori furikake, wrapping in roasted laver seawed, wrapping in nori, scrambled eggs, and chunks of leftover salmon.
Things that have not tasted good in an onigiri: Peanut butter, sweet potato (though this might work better another way), shredded carrot, jellies…
Anyway. Then you fill your onigiri by hand. I dip my hands in water (salty of not), spread a pancake of rice in my hands, fill the middle, fold in half and lightly press closed. This makes the triangle kind, and I suggest you watch a youtube video to see how to do that, cause it’s hard t explain. You can also use a mold, which I do sometimes too, especially for just furikake onigiri. Again, I’d you tube it. rinse hands a little between each onigiri so the rice doesn’t stick to them.
AND THAT’S THAT! Not complicated, tons of varieties, and something you kinda just know how to do with your hands. They’re a little exhausting if you’re making a party sized amount by hand squeezing, but it’s crazy easy to make one or three.