The good news is: it's usually easier to gain weight than to lose it. Here are some steps to help.
Consult your doctor before you start any new diet/exercise plan. Make sure to discuss the plan and take care of any concerns. Consider visiting a nutritionist.
To gain weight, you don't just want to gain fat. You also want to build up muscle and strengthen your heart. So lift weights, walk or jog, climb stairs, and swim or play sports. Try to exercise at least four times a week for at least 20 minutes (more is ideal but build up slowly if you are currently a couch potato).
Pay attention to the food pyramid. You want a balanced diet with food items from every category.
If you haven't already started, learn to read nutrition labels and get in the habit of reading the label on every food item you buy. Key things to watch for are serving size, caloric content, grams of fat, protein, fiber and vitamins. A healthy diet is largely carbohydrates, some protein, a little fat and enough of the daily vitamins, with a reasonable amount of fiber.
The most calorie-dense items are meat, dairy products, eggs, nuts, and "junk food" snacks. Eat these to gain weight, but don't overdo it, especially with the junk food, which has lots of calories but no nutrients.
Know the difference between "good fats" and "bad fats." According to current research, unsaturated is good, saturated is bad, and "trans fats" or hydrogenated fats are very bad! Different kinds of fat have varying effects on your cholesterol, which influences the health of your arteries and your heart. Examples of good fats are avocado fruits, the natural oils in some types of fish (such as salmon and tuna), flaxseed oil, and olive oil. Try to avoid or minimize the fats found in products like coconut cream, butter, and Twinkies.
Don't forget to eat your fruits and vegetables. Even though most of them are low in calories, they're full of nutrients you need, like fiber, vitamins and minerals.
Don't overeat. It will make you feel sick and might damage your body. Eat as much as you feel comfortable with (maybe just a few extra bites), and no more.
Don't try to gain weight too quickly. Your muscles will build up and your body will eventually store the extra calories as fat, but it will take time. Don't rush.
Have a target weight in mind and cut back on the calorie intake when you reach your target.