How can a 39-year-old male who is six feet tall and weighs 119 pounds increase his weight?

Consume more calories than you burn. It also depends on how much you want to increase your weight. If you are looking at a significant amount, then my suggestion is eating a lot of carbs. For example, eat a bagel every morning (if you don't already), and eat a lot of McDonald's. In order to gain one pound, you must consume 3500 calories more than you burn, and you burn about 1500 to 2000 calories a day at your current weight. Going to the gym and engaging in strictly anaerobic exercise -- that is, resistance training with weights -- is a good idea. Since you apparently have an ectomorphic body type, you will have to work harder than most to add muscle mass. Before doing so, you may wish to master your own body weight first, by doing pushups, squats, and sit-ups. If you can't do 25 flat-as-a-board pushups in one set, you're not ready for the gym. Right after you read this, drop to the floor and see how many pushups you can do. Your goal for tomorrow is one more than that! Your goal for the next day is one more than that, and so on. If at any point you are unable to improve upon yesterday's total, then you need to start skipping a day in between routines, because your body requires recovery time. The recovery is just as important as the workout. Be patient. It's gonna take a while. If you eventually get to the point where you can do 50 pushups in one set, you're doing better than 95 percent of the population. Use a similar technique for squats. For sit-ups and crunches, do as many as you can until you feel your back getting achy. Do not do any more. Sit-ups are for abs not backs! If your abs are sore the next day, great, but wait for the next day before working them. Recovery time, remember. When you build up some muscle mass in your butt, quads, abs, triceps and pecs, you'll feel better about going to the gym and pushing some steel around! Remember, be patient with yourself. "No pain, no gain" is axiomatic, but too much pain leads to its total avoidance. If you overdo it, you will be so damn sore that you will become discouraged and cease working out. So don't overdo it. Slow and steady wins the race. The above poster is correct, BUT you should see your doctor and have a good physical with blood work-up done to be sure you don't have thyroid problems, etc. Once you get the A-OK then all the directions the above poster told you is right on! Good luck!