Get the best possible quality fish that you can. Go to your local fishmonger (if you have one) and ask what's good and sushi quality. You might end up with salmon, you might end up with yellowtail. Make sure it's the best quality, and make sure that it's sustainable. Take it home, and whip up this simple yummy marinade!
1 tbsp sesame oil
2 tsp light soy sauce
1/2 tsp rice vinegar OR white wine vinegar
1 tsp mirin OR 1/2 tsp honey
Now put it in a zipper bag with your fish, push all of the air out (but don't squish the fish!) and seal it. Store in the fridge for an hour or so while you prep your veggies and cook your rice! I recommend using a short grain sticky rice or sushi rice - it will hold together really well so that you can get a bit of everything in each bite.
Some of my favorite vegetables to prep are crunchy bean sprouts (marinated using the same recipe as above), lightly steamed spinach dressed with salt and sesame oil, and creamy avocado. The sky is the limit - use your favorites and make it sing with color and flavor! Then, take your fish out of the marinade and slice is against the grain delicately. I recommend 1/4" thick slices, using a sharp serrated knife and cutting in one smooth, slow motion. Don't saw back and forth - if you can't cut with one slice, get a sharper knife!
As a "dressing" or topping for your bowl, you can use good quality soy sauce,or mix up another little batch of the marinade recipe. Sriracha or gochugaru (spicy Korean chili powder) are a great additional of heat, and spring onions (scallions) add a fresh bite to the dish! Enjoy!