With all the "Do this" and "Don't even think about doing that!" are we actually going to get to eat something at the sushi bar? Yes, yes we are. But like all the dos and don'ts for how to eat, there are even a few about what to eat. They're not hard and fast rules, just some suggested guidelines. Like Maria with the Von Trapp Family, let's start with the very beginning...
It's a good idea to eat sushi from the lightest, mildest fish and work your way up to the stronger, oilier fish. That way, you avoid the risk of strong fishy flavors loitering about your palate and tainting subsequent flavors. Gari can only do so much to cleanse the palate.
The Japanese term for generic classes of white fish is shiromi, which means "seasonal white fish." You could ask the sushi chef for shiromi and he would give you whatever white fish is in season and freshest that day. Most likely, you will see the word hirame on the menu, which basically refers to any type of flat white fish like fluke or even flounder. However, technically, flounder is karei. Many restaurants serve halibut as hirame, but again, technically, halibut is dohyo, which I've never seen on a menu in my L.A. sushi adventures.
Confusing? In the end, they're all shiromi. And since they're all light and mild, do shiromi as sashimi (fish sans rice) to start.