1. know how to lose weight (really) 2. know how to eat 3. know how to workout
Here, if you want to try fasting, which I do recommend, give this a shot. http://www.leangains.com/ or do the Warrior diet (one huge meal a day). A 16/8 fast is great. Heck, I sometimes even do an eat-stop-eat way and skip an entire day after I had a cheat day.
If you want to lose weight, it isn't about a crash diet, it most DEFINITELY is not about cardio and will never, ever be about low fat.
Here it is, in a nutshell, to lose weight and maintain decent muscle mass through life.
Eat whole foods in these categories only. Veggies, organic/grassfed meats (chicken, beef, fish etc), organic eggs. Some fruit (don't go nuts) and, only if you tolerate it, very little dairy.
Some rules: every meal has protein, no exceptions. Fat is your friend. Carbs from veggies and fruit only.
Now, exercise like this: Hard, very, very hard. And short. Think lifting big weights in a complex manner like squats and deadlifts. If someone says you should be doing front shoulder raises, run.....
And sprint...bike REALLY fast for a short period of time, or jump rope really fast...something intense and short. Kettlebell swings in the 150-250 range would be great.
Do that a couple of days a week (2-3) there are awesome programs out there so don't think too much about this, rather just do it. 5x5 programs, strong lifts, or even the leangains guide has some.
There are awesome ways to get lost in details in all this, but that is like a 10 minute guide that I wish someone gave to me when I was 14. It literally took me this long to unlearn all the damage the high-carb, low-fat movement in the 80s did to us.