You need to know what sushi really is. It refers to the rice used, not the fish.
Sushi comes in many styles. The most common type of sushi ordered is nigiri, which consists of a small, bite-sized rice log with a slice of fish on top.
Tekka-Maki is the seaweed ring around sushi rice with fish in the center.
My favorite (and hardest to get) is Chirashi, which consists of a bento box with a variety of fish, vegetables, and relishes on a bed of sushi rice.
Oh yes, the little green dab that comes with your sushi dish is wasabi. Approach with extreme caution. I love it, but it can get your head lit up and your nose running. It's comparable to horseradish on steroids.
You need to understand the whole personality behind Japanese cooking. Japanese food has complex, yet delicate flavors. If you're worried about spicy food, avoid Korean. (I love it, but it can be too spicy for some people.)
Now to the fish: the fish is typically raw. My favorite, maguro (lean tuna) has a delicate, sweet flavor. If you closed your eyes, you could not tell that you are eating raw fish. If you are squeamish about eating raw fish, I recommend ebi, which is cooked shrimp.
There are two basic types of fish used in sushi: lean, and oily or smoked fish. The oily fish like mackerel, or the smoked fish like eel I do not care for. They have a fishy aroma and taste to me.
Tuna, yellowtail and other white meat fish are very good and not fishy. If you feel adventurous, you might try tako (octopus) and ika (squid). Both have a resilient texture and a wonderful, non-fishy flavor.
Add you your meal a cup of ocha (Japanese green tea) or sake (Japanese rice wine, and pronounced sah-kay) and you're in for a gastronomic adventure that only the Japanese skill and artistry can provide.
The bottom line is: what's good is what you like. You won't know what you like until you try it.