Probably because I have a man's height (again something I expect not to see in the majority of people here) and so I needed to eat so much when I was a kid and when the height leveled out, my eating habits didn't change.
While I was still growing I was still doing sports at a national level as well... So I could pretty much eat whatever I wanted and however much I wanted and not get fat.
Then I went to the army and after getting out I kind of just stopped doing sports for the most part. My growth also stopped at a measly 6'5 during my time in the army.
Needless to say my eating-habits did not change and I started gaining weight really fast, capping at a whopping 300lbs last christmas :P It took a few years to get to that 300lbs from my ideal weight of 230lbs but I did do some on-off exercising during that time. Had I just lazed around it would have probably happened in one year or so.
I know a lot of people who did a lot of sports when they were young but got fat quite fast once they reached adulthood and quit sports without changing their eating habits :P
On a very fundamental level if you're overweight, if you eat less calories a day you'll lose weight, if you're in the mid 200's you can drop 10-20 pounds in a few months before you even start exercising just by eating less. Don't eat late at night, i know a lot of people play games until 2-3am which means they go from about 7 hours after dinner, so people tend to snack, late night snacking by gamers is a huge weight gain cause, either eat later or just tough it out, or eat something that is really healthy, like low fat yogurt or a banana just to give you something to snack on before you go to bed.
If you want to get a little more serious stop drinking soda and substitute it for water 100%, jog in the morning or walk (if you're fat walking will do more for you because it's easier on your body, if you haven't exercised in a while and if you try to run a mile you can make yourself passout or throw up which isn't fun and it's not good for you. Get in your car, measure a mile around your block (or if your EU .... idk this post isn't really directed at the OP just a general post since it doesn't seem like the OP is american) and then every morning walk that. Go on a stricter diet of less than 2000 calories, pref the low 1000's doesn't matter what you eat as long as you stay within the calorie limit. This should get you some results without going too hard as well.
If you want to go HARD, grab a gym membership and grab the book (http://www.amazon.com/The-New-Encycl.../dp/0684857219 by AHNOLD i knew nothing about working out, and this book is AMAZING AMAZING AMAZING AMAZING AMAZING for figuring out what work outs target what muscles, and it has guides on what type of workouts are best for you and your body type, when you should work out, diets, and schedules, honestly an amazing book, even though it says "Bodybuilding" only part of each chapter addresses bodybuilding competitions, most of the book is in a readable format for normal people.) Get some great cardio in every day even if you don't go to the gym, and go on a rigid diet targeting what you want to do, build muscle eat protein etc. Lose weight, eat chicken and salad for months.
What I did was i hit the gym 6 days a week and worked out, i ate chicken and salad for about 4 months and i stopped drinking soda. I lost about 30 pounds in that time span and i got a lot healthier.
The biggest thing i can say about gymming and dieting is that you want "Cheat days" where you don't go to the gym or eat mcdonalds, but sometimes one day turns into 2 and then all of a sudden you haven't been to the gym in 4 days and you're losing progress. Just keep doing it, keep a schedule and keep up with it, don't cheat, work hard and stay focused. I worked at night, then went to the gym, slept through most of the day played video games all afternoon and then went to work at night and i did fine.
You don't have to drop everything and work out throughout the day each day. I usually do 30-60min a day, six times each week. Leaves more than enough time for other stuff, for example amusements and different leisure activities. While I've just lost 12lb since January, I've manufactured a considerable amount of muscle as I make a point to join together weight preparing with cardio