You might be a little underweight, the real questions are are you eating well and exercising enough. Be your own trainer and nutritionist, the basic rules to healthy living are no soda, no fast food, lots of chicken, fish, fresh (not canned that’s bad) fruits and vegies, whole wheat stuff, yogurt. Not so many cakes and donuts and cookies.
Basic common sense for eating, you just need to enforce it. Get all the bad stuff out of the house right away, just toss it or give it away while your still motivated. No alcohol if that applies. Some juice but make sure its all natural, because the ones that aren't are loaded with sugar and a leading cause in diabetes in children and adults. Lots of water, 3-4 liters a day, not all at once, but spread it out. Its really easy to down a liter or two while your watching tv or on the computer. Also drink some after you eat the bigger meals of the day, it helps your body pick out the nutrients in food when your food is more spread apart. You also need to stay away from any sports drinks unless you’ve just had a long workout and then you need to limit yourself.
Most atheletes mix Gatorade with water because its two complex for your body to get all the nutrients out of. It needs to be diluted.
These rules may be hard to adapt to at first so start with cutting out the things you know you can live without.
Then work from there. Remember these are perfect eating habits, it can take you a while to work into it. Another important point is getting the right amount of sleep every night. You need 8-9 hours of sleep each night for your body to function correctly. Set up a routine for yourself and go to bed at about the same time each night.
As for exercise, lots of cardio. Find something you like. If you have never exercised much before, start with walking 30 minutes a day at a brisk pace. Move on to different terrains that include hills and increase the time spent. After a week or two, try jogging the route. No matter what find an activity you enjoy. Walking to jogging is just an example, try hiking, rollerblading, biking.
Once you get into a regular routine, if your not happy with the level of difficulty, try signing up with a gym. Spinning, Pilates or kickboxing classes are great. It will also introduce you to working with weights. Talk to a trainer for instruction before using any weights. A cheap investment is a balance ball, it works you stomach while your just sitting on it, like at your computer and its great for posture. There are hundreds of exercises you can use it for for your entire body.