Too skinny, want to firm up muscles; just retired from sedentary job?

Exercise. Congratulations on your retirement and your decision to improve your health! Start by reviewing your dietary choices. You know what's healthy: fruits, veggies, non-processed foods, lean proteins. Avoid fad diets. After meals, go take a walk. Start slow for short distances, then build up speed & duration. After you return home, do some body weight exercises. Check out https://goo.gl/2hVo1k.