Sushi is just pressed rice. Sometimes seasoned - after cooking - with rice vinegar and sugar (50:50 ratio; 1/3 to 1/2 cup is enough for 3 cups cooked rice) or salt (lightly) to help preserve it in hot weather, since rice hardens in the fridge. But you can use the rice without any seasonings at all, if you'd like. "Fresh" sushi is common for home meals.
Use short or medium grain, good quality rice. STEAM it, don't boil it. Sushi rice is cooked a little firmer than dinner rice. I often make the rice ahead of time, season it, press it into 1/2" high rectangles with my kit, then wrap it up for the freezer. Makes it easy to defrost and MW to soften, let cool a bit and make
nigirizushi.
A plastic kit for pressing rice, as Happyflowerlady shows above, is standard. Try to get toasted nori (seaweed sheets), they taste better IMHO. Toasted sesame seeds and "wakame gomae" (seaweed salad), along with pink pickled ginger slices, are nice additions to the plate if you don't care for wasabi, which is mostly just horseradish powder dyed a virulent green. True wasabi isn't as pungent - spicy-hot rather than bitingly hot - is creamy-white, and has a lovely floral scent, but it's expensive and hard to find.
ANY raw fish used for sashimi should be frozen first. You cannot import any raw fish into the USA without it first been frozen.
All fish, salt or freshwater, have parasites. NEVER EVER EVER eat any raw fish that a friend has helpfully caught, no matter how fresh it is, as sashimi.
"Sushi grade" fish means more than just a good cut - it means it has been properly flash-frozen for at least 48 hrs to kill any parasites.
You can use anything you'd like for fillings/toppings. Spam! Cooked shrimp! Lox! (my spouse's personal favorite, btw) BBQ/teriyaki chicken breast!
My personal favorite is sushi rice mixed with chopped shiitake mushrooms, finely diced carrots, and petit pois (baby peas), stuffed into tofu skins called
inari. I like the unseasoned skins but they are harder to find than the seasoned ones, which are soaked in sweet soya. They comes in plastic packages and also cans - in fact, Walmart sells them!
They can be a little tricky to separate into "pouches" - fingernails help, LOL. Just work apart gently - a small tear here and there isn't a big deal.
One can be very elaborate about inarizushi but I don't bother when it's just for the two of us
So be brave, everyone, and have some fun with it!