If you've been skinny all your life, and have trouble putting on any kind of weight, working out can seem pointless. However, it's simply a question of getting the right kind of training in place, combined with some other key lifestyle adjustments. Regardless of whether you're male or female, getting these basics in place will help you get the body you want. While putting on size or definition takes time, it can certainly be done, even from the comfort of your own home. You don't need to be strong from the start. You just need to start.
Bodyweight circuits are a great place to start if you haven't done much training before. Using your own bodyweight for resistance, you will gradually build up your strength and add some lean muscle mass, helping you to get a little bigger and more defined. [Reference 1] Here is a circuit you can do three to four times per week to start building up your body. Perform 10 pushups, 15 squats, 15 Russian twists (sit balanced on your hips, then twist at the waist to either side), 10 triceps dips, 20 walking lunges, and a 30-second plank. Do as many rounds as possible in 15 minutes.
Get Some Weights
Free-weight training is a highly effective method for adding size and strength to any type of body, but will be particularly noticeable on those with a "skinny" frame. Once you've gained enough strength to deal with bodyweight circuits easily, weights are a natural progression. [Reference 2] Getting a set of dumbbells or a power rack to use in your yard or garage will allow you to really increase the intensity of your training, without requiring you leave home. If you're female, don't worry about getting "bulky" -- most women don't have the physiological or hormonal profile for that to happen. Instead, you will see improved muscle tone with a small increase in lean muscle mass. Two upper body workouts (made up of the bench press, bent over row, biceps curl, overhead triceps presses and lateral raises) per week will increase the size and strength of your arms, chest and back. Two lower body workouts (with squats, deadlifts, reverse lunges, single leg squats and calf raises) will increase the size and strength of your legs, buttocks and lower back.
Fast, Intense Interval Training
Interval training has gained a lot in popularity over the last few years, as it is highly effective for lean muscle development. Interval training is best done over a 10 to 20 minute period, alternating periods of intense work with slightly shorter periods of rest. Training in this way stimulates your fast-twitch muscle fibers, causing an increased rate of lean muscle development, particularly when compared to cardio performed at a steady pace for the duration of the workout. [Reference 3]
Fuel Your Body for Muscle Growth
Getting in growth-focused workouts is a great start when you're trying to get a little bigger or more defined. However, your workouts are only one piece of the puzzle. You also need to make sure you're eating a significant amount -- 18 to 20 calories per pound of bodyweight, or even as many as 25 calories per pound if you are very skinny and are having a hard time putting on any size. [Reference 4] It's also key to make sure you're sleeping enough, and to be consistent. Your body simply won't grow if you're only eating large enough meals and working out once a week.