Higher metabolism helps (I know, I have it). But only so much. I have learned a few other tricks from my parents (not that I need them, I've always been skinny), that I don't think are as well known as "exercise" and "don't eat junk food".
1. Eat varied and healthy food. This is more than just stopping to eat at McDonald's etc. It's going out to grocery stores and buying raw supplies for actually cooking your own food. Don't know how to cook? Take this lesson from my sister: "As long as you're literate, you can cook" (Buy a cookbook a varied set of recepies, or find a good site online).
2. Make sure you plan ahead for three full meals a day. Don't go around all day "snacking", don't wait until you're hungry before planning your next meal/cooking. I personally like the 8-1-8 schedule, with an obvious flexibility of an hour or two (To be honest, I've yet to properly master this scheduling of meals things, but I still don't go around snacking, it's just not part of my 'culture'). Don't be afraid to have one snack between meals, but by snack, I don't mean finishing a bag of chips/candy/candy bar. Eat a fruit, or if you had a light lunch, make a sandwich.
3.Finally I'm not sure how true this is, but I heard of a study where they realized during the study that ppl from the USA were taught since childhood to "finish the plate" irrelevant of whether they were full or not, while ppl from European countries weren't (Obviously a generalization of both sides, even if it was a study). Even if this study isn't true, I think it's still important to note the obvious: If you eat more after your body tells you that it doesn't need any more food for now, and you do this every time you eat, you are obviously going to gain some weight. My parents always told me to eat only as much as I wanted/could. Sure they would encourage me take "one more bite", if I was obviously trying to finish my food in a hurry to go and play, but I was never forced to eat. his is especially important if your'e actually going to follow the previous two advices. It's always better to cook too much than too little, but don't finish food just because there is food left over, make a box of "leftovers" for quick reheating one night, when you're too tired to cook.
Excersise (even just an hour/half hour of extra walking) obviously also helps a lot. And yes, once again, high metabolism helps, and low metabolism sucks. But eating properly and exercising will ensure that you stay healthy.