Brush grill pan or 10-inch skillet with oil. Heat pan over medium-high heat. Sprinkle taco seasoning mix on chicken breasts; cook on grill pan 10 to 12 minutes, turning once, until juice of chicken is clear when center of thickest part is cut (at least 165°F). Remove from pan. Let stand 5 minutes. Cut chicken.
Heat taco shells as directed on package. Fill taco shells with chicken, tomatoes, lettuce and cheese.