Congratulations with your progress so far. I used to get a bit fatter as well, not FAT, but if I continued living the way I did I might have been fat right now. My secret diet is no diet at all, just eat regularly and healthy. The secret to losing weight is by doing so with something that you have fun doing. Don't lose weight on a treadmill, go do something that you enjoy to do, not just to lose weight. People who lose weight on a treadmill start gaining weight again once they reached their goal because they stop exercising, since it simply isn't fun.
I started with karate a year ago, I didn't lose much weight at all, but my belt (the one to keep my pants up
) still feels loose when I wear it at the first hole. I used to have it set in the 3rd or even in 4th hole for more comfort. I didn't need a belt back then, but now I can have 2 fists in between my pants and my body and still have space left.
Really, martial arts with a very strict and experienced sensei / sifu is a good way to lose fat and get some condition.
Of course if the idea of martial arts doesn't appeal to you, you can try another type of sports, maybe a team sport? But you never know whether you'd like it if you never tried it. I'm sure you can take a trial class for free to see if it's something for you at the local dojo (if you live near any).
By the way, build up some muscle as well. More muscles burn more fat. Do some weight training on the side of the other sports you'll pick up.