My HEB doesn't have sushi-grade fish. I have to go to central market. They sell frozen cuts of both tuna and salmon. Let it thaw in the fridge for a few days. Then all you do, for poke, is dice it, toss it in some soy sauce and maybe some sort of hot sauce of your choosing, and throw it in a bowl with whatever the fuck you want. I usually do brown rice, cucumber, avocado, jalapeno, green onions, and some sesame seeds.
For sushi, it's a little more of a pain in the ass. I'd watch an instructional video. You need one of those bamboo rolling pads. I suck at making sushi rice so I let my wife make the rice. Then you have to get the seaweed wrappers and lay a thin layer of the rice on the seaweed, and then fill it with whatever you want - typically much of the same shit as above - cucumber, avocado and jalapeno, maybe a spicy mayo. Then roll them bitches up. there are a million recipes out there, but basically it's just seaweed wrap, rice, fish and fillings, roll it up, and then slice it up.