This might go against the grain of what the local mantra is (not that I disagree with it, per say), but when I finally got it together and started to lose weight, I started with exercise. The diet modification followed (mostly due to not wanting to ruin my hard work by overeating).
I started with p90 (NOT x), then moved up to Insanity. p90 is good for people like me who beforehand (not counting some martial arts, etc) have never really done any cardio. Its just enough to feel the difference and see the scale budge... which helps you stick with the harder stuff that really makes the process work.
Also, since I could never do actual long-term diets, I *did* do a 13day "metabolism boost" diet thing... though maybe that only helped in getting me to eat better foods (and better portions), though I did feel better after the first week of annoyance. I think that was after I did p90 and basically the first time I (consciously) altered what I ate... not counting having given up soda, etc for the most part.