The advice about cutting out soft drinks is a good one. The amount of sugar those things is pretty ridiculous.
I used to be a skinny kid all throughout my childhood, but my metabolism slowed noticably around when I started uni. I'm still in a normal weight range, but I miss those days when I could eat whatever I liked and play games all day and still be able to fake a "slim build". These days, there's a pretty direct link between the time I put into excercise & what I eat, and how fit I look (I'm by no means thin and muscular, excercise just keeps me at "only slightly squishy").
It's definitely not immediate, and it's hard to really notice the results, but if you stick to an exercise routine for a while (over 6 months), it shows when you look back at photos of yourself and compare them over time. I guess the toughest thing is to stick to it. But take some progress pictures, and if you get discouraged further down the track, have a look at them, they should show that your efforts are making a difference.