Okay, tuna probably isn't the best choice for a fish/seafood newbie since it's not exactly subtle. But I have this recipe on hand, so this is the one you get first. It's from epicurious.com. I've served it as an appetizer for weekly dinner a couple times. It comes together really quickly and it's always gone over very well.
Peppered Tuna Skewers with Wasabi Mayo
2 Tablespoons wasabi powder
1-1/2 Tablespoons water
1/2 cup mayonnaise
1 lb. tuna steak
2-1/2 Tablespoons soy sauce
Fresh ground black pepper
Combine wasabi and water to make a paste. Add mayo and refrigerate at least 30 minutes. It takes this long for the wasabi flavor to come through — an early taste-test will not give you an accurate idea of the flavor. Marinate tuna in soy sauce for the same 30 minutes. While that part is sitting, thread pickled ginger onto short skewers (I just use heavy-duty toothpicks). The original recipe suggests serving on a bed of watercress for pretty.
After the 30 minutes are up, drain the soy sauce from the tuna and pat the tuna dry with a paper towel. Add pepper to tuna. Heat vegetable oil over a medium-high flame until shimmery, and quickly sear tuna (about 2 minutes on each side for a good, 1-2" thick tuna steak — you want it to be rare in the center). Remove from heat, carefully cut into 3/4" cubes (the tuna will want to flake into its own shape, particularly the parts that are the most cooked) and add to ginger skewers.
Serve with wasabi mayo dipping sauce.
Note: You will have lots of extra sauce. I've used it up as another fish dish. Take filets of whatever fish you like and place them in an oiled baking pan. Spread wasabi mayo over the top in a thin layer, then sprinkle seasoned bread crumbs/panko/crackers/whatever over that. Bake. (I refer to Joy for baking time/temp for whatever type of fish I'm using. If it's a thinner, lighter filet, you may need to switch it over to broil for the last couple minutes to get the top nice and crunchy.)