Gaining weight is a two step process: 1) eat more and, 2) exercise more. When you start eating and exercising more, your body will use the extra energy to build muscle. Combining both steps is essential as simply eating more will only lead to fat rather than muscle gain.
1. Create a Calorie Surplus. A calorie surplus is a state in which you eat more calories than you burn. It is essential because gaining muscle is a very energy intensive process. When combined with strength training, your body will take the surplus energy and use it to build muscle. Use the calorie calculator to help you figure out how many calories your body burns each day. Once you know what your body needs each day, add 500 calories and watch your weight increase. To ensure that the calorie calculator is accurate, keep track of your weight. Make sure your weight is steadily increasing and adjust your calorie intake accordingly if it's not. Don't increase your intake too drastically or you will start to put on body fat. Your body builds muscle at a slow pace so simply eating more won't necessarily lead to more muscle mass. Remember that if you're not gaining weight, you're not eating enough.
2. Lift Weights. The second step in gaining muscle mass is to engage in some sort of strength training routine. The most popular form of strength training is weight lifting. When you lift weight, or do any form of intense exercising, you cause tiny tears in your muscle tissue which get repaired during rest. This recovery process is what makes your muscles bigger and causes weight gain. Strength training is only effective if you constantly overload your muscle by either lifting more weights or completing more repetitions per set. If you don't overload your muscle, your body will have no need to make you stronger or build bigger muscles. See a sample full body and split routine.
3. Proper Post Workout Nutrition. After a workout, the body has two main priorities: 1) replenishing energy stores that were depleted during exercise and, 2) starting the recovery process. During exercise, your body uses a lot of glucose (sugar found in the blood) and glycogen (storage form of glucose found in the liver and muscle tissue). Once you're done exercising, your body wants to return blood glucose and liver/muscle glycogen levels back to normal. Your body can do this in two ways: 1) it can make glucose and glycogen by breaking protein (either from a post workout protein shake or by breaking down muscle tissue) or, 2) it can replenish energy stores from simple sugars found in a post workout shake. Drinking a shake that is made out of 100% protein will cause your body to use the protein to replenish energy stores rather than repair muscle tissue. Sugar spares the protein so it can be used to help you gain muscle. Including simple sugars (honey, ice cream, fruit, chocolate milk) in your post workout protein shake will help your body start the recovery process quickly and efficiently.
4. Eat Small, Frequent Meals. Increasing your calorie intake isn't always easy. Many people make the mistake of eating larger meals rather than more frequent meals. Both options will increase your calorie intake however eating smaller, more frequent meals will help your body better utilize the calories for building muscle. Your body will absorb 100% of the calories you eat. Unfortunately, your body doesn't always have a need for 100% of the calories you eat leading to excessive body fat. If you eat a huge meal, your body is much more likely to store some of it as fat. Eating smaller meals will give your body a more manageable supply of calories and help you avoid excessive fat storage. Try to eat every 2-3 hours.
5. Eat Healthy. Gaining weight shouldn't be viewed as a blank check to eat anything you want. Keeping your diet clean will help keep your gains clean. Avoid unhealthy junk food such as: sweets (doughnuts, muffins, candy, cookies), soda, fruit juice, sweet tea and fast food (remember that simple sugars are find in conjunction with a post workout shake). Instead, eat: fruits, vegetables, brown rice, 100% whole grain flour products (bread/pasta), beans, nuts, legumes and lean meats.
The Bottom Line
You should view gaining weight as a long term goal. The body builds muscle at a slow pace. If you stick with the steps outlined above and create a healthy lifestyle around them, you will pack on clean weight rather than fat.