Contrary to conventional wisdom, there are no longer more fish in the sea. Many fish—bluefin tuna, for example—have been pushed to the brink of extinction, because they are delicious and we can't stop eating them. Given that fish is an incredibly important part of most American's diet, and that we haven't yet found a way to farm fish at any scale, the best thing to do is to eat fish consciously. But there are lots of kinds of fish, and it's hard to remember which ones are safe and which aren't. And where and how fish are caught can affect your decision about whether to eat them.
This helpful illustration (click here for a large version) from Information Is Beautiful and The Guardian should help you with the conundrum you face at the fish counter. Divided into "Yes," "No," and "Maybe" and then subdivided by ocean, it's a simple interface for identifying the common fish you should gravitate toward (for ease of reference, you should probably skip the maybes, too.)
Of course, new research has found that even the small fish that make up a lot of the "Yes" section aren't as smart a fish decision as was once thought. It's less about individual fish then in making sure you have a better understanding of what you eat and where it comes from. And it's always helpful to have a reminder to not eat one of the last tuna.