Ill outline what your more than likely going to go through. Its long, but worth the read.
1. First thing you have to do is decide your gonna get into shape then do it. You already know its your time to get into shape, so all you have to do is buckle down and do it.
2. Kill the Big Macs and pizza, at least for the 1st 6 weeks. The biggest motivator is going to be the results you see in that first couple weeks.
3. We need to determine how many calories you need to eat, 11 stones is approx. 154 lbs. Your healthy weight is 122 lbs, in reality, with NO muscle, you should be able to lose 32 lbs. That number is theoretical, don't shoot for it as a goal, just as a guideline. To maintain your current weight, you need to eat 2500 calories a day at a moderate activity level, eating 500 less than this will create a defecit that will allow you to lose 1 lb of fat a week in eating alone. Any other defecit you create with exercise will increase your loss. Don't try to lose more than 2 lbs a week though. Initially you may lose more than that a week, its normal and will slow down.
4. The first week of eating like this you may feel extremely hungry at times. Eat a piece of fruit or drink some water, this does go away after a few days.
5. Exercise is key. You should incorporate weight training and cardio for fat loss.
http://training.fitness.com/showthread.php?t=3970&page=4
I give a basic starter weight training workout here split into 2 days (upper and lower body). Every lb of muscle you build burns more calories for you doing nothing. Cardio you should perform 4 times a week at least 30 minutes a session (45 is better). If you would rather do HIIT (search the forum for this) 5 minute warm up, 20 minute session, and 5 minute cooldown is sufficient enough.
Fat loss isn't just about diet and exercise, thats where most people fail. They think if they eat little as possible or exercise every free minute that they will get the results they want. They end up burning precious muscle and causing metabolism to go to a crawl. Thats the name of the game, your metabolism. You have to get it into overdrive. How? Consistent exercise, proper diet and 5-6 small meals a day. Eating 5-6 small meals a day keeps your metabolism high thoughout the day. Fat isn't burned at the dinner table or in the gym, its burned while your metabolism is in overdrive all day long.
Try
www.fitday.com, it is a free online dietary journal that tracks calories, protein, fat, and carbs.
Do this for 6 weeks and I promise you won't be dissapointed. After you see what it takes to lose weight, we can work on the finer points to boost your results (protein supplementation, macronutrient distribution), just try to keep your fat intake limited to 20 percent of your total calories.