I did things in this order:
1) Cut out all refined sugars from my diet. If I want something sweet I eat fruit, or something naturally sweetened w/ evaporated cane juice or something. Moving away from starches helps A LOT too, so eat things with whole grains rather than white flour, and eat brown or wild rice rather than white, etc.
2) Cut out all fast food - that shit is terrible for you. Getting that crap out of your body will actually improve your athleticism FAST... our bodies don't process the shit they put in that food very well.
3) Cut down on fatty foods. If I can choose between grilled or fried/crispy chicken... pick grilled, etc. And I don't eat much cheese these days, and when I do it's usually feta.
Those things will make a HUGE difference all on their own.
Of course once you're actually feeding your body food that it can use for energy and nutrition rather than stuff that just makes you full there's still the matter of thermodynamics... eat more calories than you metabolize and it's gotta get stored someplace right! :)
So boost your metabolism... some foods can even help with this. Hot peppers have a chemical in them that actually acts as a natural metabolic booster. Exercise helps the best of course.
So once I changed my eating habits I started doing other things... like push-ups, pull-ups, etc. Started out only doing a few, and worked my way up. It's really discouraging when you start out, but after only a few months I was able to do 300 push-ups in sets of 75 without too much trouble, and 100 pull-ups in sets of 25.
What helped more than anything was biking... I LOVE mountain biking, almost most as much as I love skiing. When I'd get bored, and subsequently tired, instead of sleeping in the middle of the day I'd go out and ride my bike for a couple hours.
So biking, running, hiking, skateboarding, skiing, whatever... you just start doing it enough and instead of being an "outing" it becomes just one of the things you do regularly... and you'll find they start to replace all the bad habits you had before.
LOL... that was quite the diatribe, and in a boot fitting thread. :)
Though seriously if you want more info or have more questions, hell I probably even have some pics from both times in my life if I look hard enough just kick me an e-mail.
Cheers!
-Nathan