Preheat a cast-iron skillet to medium-high. Add the ground beef and cook, breaking up with a wooden spoon, for 1 minute. Stir in the garlic, chili powder, oregano and cumin; season with salt and pepper. Cook, stirring frequently, until the meat is browned, about 5 minutes. Add 1/2 cup water, turn the heat to medium-low and simmer until the liquid is reduced by about half, 2 to 3 minutes. Remove from the heat and cover to keep warm.
To make the tacos, spoon 2 to 3 tbsp. of meat into each shell and top with the desired accompaniments.