2 Answers

Being skinny doesn't indicate being weak. The best advice would be milk shakes and you can buy a protien powder to add. Another idea would be eating regular meals and having a meal replacement drink inbetween...something like Boost. Don't worry ,in most cases the weight will come on as you age.

If you want to add muscle tone to your whole body, add more calories to your routine meals.  More poultry, fish and some red meats cooked.  Doing this, I'd suggest a routine workout with weights to build the upper body, including the abs as well as the pecs and more muscle to the legs but not the butt.  lol    To gain weight, add 500 calories a day and you should gain 1 lb per week.  Don't add too many high calorie foods or you'll regret it later in life.  Right now, you are 'blessed' with being underweight while most of us are fighting. to get rid of weight.  You may also want to start a swim routine which will help firm up and tone the body. Good luck and good health.