Step 1. Cook the meat: In a skillet over medium heat, cook the ground beef or chicken until browned. Drain excess fat and add taco seasoning according to the package directions.
Step 2. Prepare toppings: Dice the onion and tomatoes. Shred the lettuce and cheddar cheese. Slice the avocado.
Step 3. Warm the shells: Heat the taco shells or tortillas in the oven or microwave as directed on the package.
Step 4. Assemble the tacos: Fill each taco shell with meat, followed by onions, tomatoes, lettuce, cheese, and slices of avocado. Top with sour cream and salsa.