Your first step is to figure out how many calories you burn each day. You can use the Daily Calorie Needs Calculator. Remember that this calculator gives you an estimate. It’ll take a week or two for you to get a better number. After you’ve used the calculator, 500 more calories than you burn each day. For example if the calculator estimates you burn 2000 calories per day, you need to eat at least 2500 calories.

You may need to repeat the above step for a few weeks before you do start to gain weight. You can also make a huge step up in calories (example: going up 1000 calories instead of 500 during the first week). This will help you gain weight faster but you risk gaining more fat stores than muscle weight. This shouldn’t be too big of a concern if you are the type that has trouble gaining weight (since you probably have a pretty fast metabolism).

Step 3
Don’t use the excuse of having a fast metabolism to eat any food you want. Even skinny people can get sick from eating the wrong kinds of food. You want to fill your diet with healthy foods such as complex carbohydrates (brown rice,whole wheat breads, fruits and vegetables), unsaturated fats (mostly vegetable sources such as oils, nuts and veggie spreads) and lean sources of protein (fish, eggs, poultry and some cuts of beef). Since you are trying to gain weight don’t be afraid to occasionally consume some “junk food”.

Step 4
Remember that gaining weight takes time. You have to be consistent with your calorie intake. Its sometimes easy to skip a meal but when you’re burning calories from exercise on top of a fast metabolism, that one meal could be the difference between gaining and losing weight.
Step5
You can get more help by reading and posting to the diet and exercise forums that are linked below. good luck!