Fish is incredibly easy. Pick a firm white fish. Some like tilapia, and that's perfectly fine, but entry level. A wahoo or mahi-mahi-like fish is better. Drizzle with a tiny amount of oil (just enough to keep it from drying out), then sprinkle it with chili powder. I do it pretty generously, but do it to taste. Bake or broil until flaky. You may want to go until it's just a little bit firmer/drier than you would if you were serving it by itself. You want it to stay firm and hold its shape when you cut it into chunks.
Cut it into taco-size chunks. Serve.
Opinions vary, but I think fish tacos should be in soft corn tortillas--not hard shells, and not flour. Do it with fresh shredded cabbage (either plain raw, or with a cider vinaigrette), avocado slices, lime wedges, and chipotle mayo.
My go-to taco sauce for ground beef or chicken tacos is Cholula. For steak tacos, I like a salsa verde, or just chopped onions, chopped cilantro, and squeeze of lime.