Im a ~30yr old male, I started putting on LOADS of weight towards the end of Uni to the point that I weighed 30st!
I could sort of get away with 'carrying' it as im 6'4, though about 20 months ago I realised that this was unacceptable and decided to do something about it. Im now down to 18st and still going downwards.
Id like to give my POV on the whole diet 'thing'. Maybe someone will find it helpful:
Temporary Diets dont work, you need to correct your eating habits permanently, dont go on 'diet' change your lifestyle.
Stop eating crap 24/7, I know it tastes good and your probably lazy like me and its quick and easy, but you need to decided whether you want to cram donuts in your mouth or have a healthy and active body you cant do both, you need to choose.
Dont be afraid to have a cheat meal once in a while, it's okay, your not in prison you can enjoy stuff occasionally, just make sure it's occasionally and doesnt become every day, like once a week and dont binge either - eat sensibly when you cheat.
In order to lose weight you need to burn more calories than you take in, it's realy that simple. Friends have said to me oh I only eat 800 calories I cant loose weight like you. I ask a little further and find out they dont excercise at all and drink 10 coffees a day (which when black have no real calories) with full fat milk and sugar.....sigh.
I used to struggle exercising - after work I was tired, busy or would make up some other excuse to do it, if you want to lose weight you need to be proactive and not think or wish about doing it - if you cant, dont bother trying to struggle yo-yo deiting, realise you are making excuses up and thats half the battle. You can ALWAYS find a way if you WANT TOO, if someone put a gun to your head and said you need to spend 30 minutes a day doing charity work or they will come and shoot you in your sleep, you will find a way to accomplish that - this is called "motivation", laziness's arch nemesis.
Find your way to exercise 30 intense minutes a day and you will be in route to weighing less and feeling healthier. I found my way by waking up EARLY and exercising before getting ready for work. This might not be for everyone, but I call anyone out who cant get up earlier and do some exercise. In the end it worked for me, because I was getting up an hour earlier and doing a preplanned series of exercises and weights in a concise time frame. A benefit of this is that exercising releases endorphin's which kick in and make the day go by faster and gave me a good mood for the rest of the day and work. Also it kick starts your metabolism so helps with your calorie intake for the day!
If you are big/heavy like I was, DONT run for cardio, use a bike, go swimming (unlikely since you will be self conscious) or use a rowing machine as you will damage your joints and tendons needlessly by putting them under serious stress, so do low joint impact cardio work until your lighter then do all that other stuff, worse thing that can happen is get injured and fall back into sitting on your bum and feeling sorry for yourself - it's happened to me!
DONT use scales to measure your weight, BMI is also a load of rubbish - it works in principle but there will be a point where you are doing the routines and not loosing any weight, because your flab is being replaced by heavier muscle and you will dispair. Instead use clothing as goals - I have goal shirts that are much smaller than I can wear, and I try these on to see progress, there is no better a feeling than buttoning up a shirt that wouldn't even close beforehand.
Thats a few pointers that helped me lose 12 stone so far, I hope it's of help to someone, it might not be to everyone's taste.