Start by thawing your fish filets. Mine come in individually wrapped filets, in the freezer section of the grocery store. To thaw, place your sealed filets in a bowl(or your clean sink) with lukewarm water. In 30 minutes or so, they should be ready.
Once that's done, take them out of the packets and lightly salt and pepper them.

In a medium-large sized skillet/frying pan, heat 2-3 Tablespoons of olive oil, on medium-high heat.
Place your fish gently(so as not to splash yourself with hot oil!) into the pan, 2 or 3 pieces at a time, leaving about half an inch in between them.
Allow to cook on one side for 3 minutes, or so, until you can see them turning white on the edges.

Then flip! And cook the other side another 2 minutes or so. Take your spatula, or spoon, or whatever you flip them with, and begin flaking the fish apart. Chop, push, rub, whatever, with your utensil, until all the fish is flaky and stringy, like rice or small noodles.

If you want your taco shells warm, heat them according to package instructions.
If using black beans, or refried beans, pour them out of the can(yeah, I'm lazy) and heat in a saucepan or microwavable bowl, just until warm.
Place all other items you might want on your taco in separate bowls, for easy assembly.

Avengers Assemble!!! I mean...tacos assemble...heheh.
Spoon out a couple Tablespoons of fish into your taco, then top with beans, cheese, taco sauce, etc.
Some other variations are sour cream, tabasco/habanero sauce, shredded lettuce, chopped tomatoes, salsa, guacamole...the list goes on and one.