Wow, wish that was my problem :). Being overweight is not good but I don't think you are asking to be fat... you want to gain a few pounds (healthy weight) and maybe increase your muscle tone right?
If your too skinny, that is unattractive as well... actually I would rather be a bit on the juicy side of the spectrum then extremely skinny (with your bones sticking out).
If you want to gain weight appropriately then I suggest eating properly, balanced meals, eating 3-4 times a day, eating healthy and snacking on healthy snacks. You need to consume more calories every day from healthy foods and choose the right types of exercise. You can also buy cretin (protein) to assist in weight gain... if you do this you should be lifting weights to assist with building muscle... because you DON'T WANT to be FAT... you just want to be HEALTHY. Take in plenty of water and dairy products (milk, cheese and yogurt... eggs are great too)... a handful of Almonds are a great snack.
The goal is to be normal and healthy... not too skinny or too fat! Here is a link to help you find out what you should weight and if your weight is considered too low.
Height and Weight Chart - height weight chart, weight height chart