I mean, yeah, eating only celery and drinking only water will definitely make you lose weight. But your life's gonna suck.
The best way to lose weight is to do resistance training. Personally, I hate weightlifting, so I prefer to recommend aerobic exercises like using the elliptical. They're not as good at weight loss, but they'll improve your heart health (which is half the battle if you're fat) and help you build endurance.
You can have a pretty crappy diet and still see great gains just by becoming more active. It's tedious as hell, but you force yourself to do it. Like going to work every day.
And if you need motivation, here's the manliest motivational speech ever given: