Get up and get moving. Take a walk after meals, go to the basketball courts and play for an hour with friends, if you do not do your own yard work start. Anything that gets your heart rate up will greatly increase your metabolism.
Also, increase your intake of fresh foods. Fruits and veggies. Anyway you like them. A simple multivitamin should be fine.
Omega-3 fish oils are great! I don't take them but I know many people who highly suggest them. If you get the capsules, I'd get a food kind... I knew one person who said the kind they bought at the grocery store give them a nasty fishy aftertaste for a few hours! No good!
To gain weight, try having protein shakes. Just make sure not to have too many where you are getting way too much protein. Try to find one higher in calories, because some are better for those who are trying to lose weight and are low in calories.
The calories you need definitely depend on your size/metabolism. Try increasing what you eat now by a couple hundred calories and see how it goes. It may be something you figure out with time.
Focus on strength training to build muscles. Too much of ONLY cardio may just make you lose weight.