There are thousands and thousands of articles that have tons of advice about losing weight, but if your problem is actually needing to gain weight it may feel impossible. It can be done with the right attitude, a healthy diet, and good exercise. Plus it can be done without getting fat or becoming overly muscled.  

If you are underweight then chances are you have a very fast metabolism (as long as you eat normally). You may have larger friends gawk in amazement as you put the food away. However, there are a few things you need to keep in mind about the foods that you eat and when you eat them.

1. You are still susceptible to high blood pressure or high cholesterol (most common). Because of this, you need to watch the foods that are bad for the cholesterol. It may seem like they will help you to gain weight, but in reality it isn't worth the health risk.  
2. You should be eating a lot. Not that you should consume huge meals, but rather that you should consume several small meals throughout the day. Your body is like a food burning engine and if the food isn't there to burn then it will start breaking down the tissues in your body (and you don't have many to spare). Aim for six small to medium meals/snacks a day, while drinking lots of healthy fluids.  
3. Just because you have the metabolism for it now, it isn't a good idea to eat a lot of junk food or to eat before bed. Midnight snacks should still be off limits (at least most of the time). This is because your metabolism may slow down as you age and often times your habits will stay the same, which may result in overweight issues later in life.  

You should consume a wide range of foods. Include foods from each food group. Eat fruits, vegetables, dairy products, grains, and meats. Add a lot of high protein foods with fats that are healthier for you to your diet. This includes whole milk, yogurt, cheese, beans, nuts, and nut butters (with peanut butter being the most popular).

You can also add healthy drinks such as 100% juices, sports and energy drinks, milk, milk shakes (yep, drink milk shakes!), smoothies (especially those you make on your own with juice or milk, yogurt, and fruit), and drinks with whey powder or soy powder (sometimes called soy protein) added to them.  

There are several types of foods that you should eat in moderation (just like everyone else). These include fast foods which are always fatty and high in cholesterol. You should also limit how many burgers, pizzas, hot dogs, potato chips, and fried foods you eat. Bacon should also remain a treat rather then a regular food. This is because all of these things can raise your blood pressure and your cholesterol.  

Exercise is important for everyone, even though a lot of us avoid it. You should do aerobic exercises at least three times a week for thirty minutes or more. Some great options are walking, biking, dancing, aerobics, kick boxing, and others. You can do many from your own home with a few items of equipment or a good DVD. You can also walk your neighborhood for free or go to a gym for a fee.

Many people who are thin are afraid to do aerobic exercise for fear of loosing weight. However, this will help your heart, and help you build muscle tone. Muscle weights more then fat and is healthy which is always desirable. It will also help you for a great heart healthy future.  

Weight training is actually good for everyone, but it scares many women. Whether you are a man or a woman your body will benefit from light to medium weight lifting. You can either do this in a gym or use a few exercises at home. Using light to medium weights will allow you to lightly tone your muscles and not give you an overly toned look. You can then work on maintaining that, or building it to the next level if you wish.

If you are going to use a gym there are usually a lot of options for toning and enjoying yourself while doing it. Ask for a demonstration of each machine before you use it so that you know how to use it. If you use free weights, it is best to have a friend there to help you should you get in trouble.  

Dumbbells are the easiest way to work your muscles at home. They can be purchased for a few dollars in a wide range of pounds. You can start small with five to eight pounds and go up to ten. As a woman you can stop there so that your toning doesn't become too much or continue on if you like the results. Weights go all the way up to a hundred pounds (sometimes even more then that) for the workout you desire.

I could explain a few exercises, but if you are interested there is a great website with moving illustrations which will do a better job.  

Yoga and Pilate's are both great options as well. They will help tone your muscles and also are really good for relaxation and building up endurance. You can buy a book, a DVD, or pick up some great suggestions from the Internet. Neither one requires a ton of equipment and can be done in the comfort of your own home. You can also take classes which are good for those who need lack discipline and need that commitment to make it.  

With the right attitude, a good diet, and lots of good exercise you can gain weight and look wonderful. You can choose how muscular you want to be and have a great body that other people envy. The key is to eating healthy foods including good proteins and fats, as well as toning the muscle and becoming fit.