I am not in contest, but I lost 50 lbs (285->235) in about 5 months when I started exercising.
Here are some tips:
1. If you are within biking distance of your job, have a safe bike route, and you have appropriate facilities at work (shower), bike to your job. Biggest no brainer ever. I now bike 9 miles each way. That's 80 mins per day of cardio even if I never see the gym. Get a basket for your bike and use it to go to store to get groceries, it not only burns weight, but you can't buy a lot of food per trip, so you'll have to get some exercise if you want to eat.
2. If you have free time at home (watching TV or lounging), go for a walk/bike/rollerblade, whatever, but get yourself out of your house. Even if you walk and don't burn much fat, you are not at home, so you aren't munching or eating, or whatever unhealthy habits you do when you are wasting time.
3. Don't keep prepared food or snacks at home. The only things in your fridge that you should be able to eat without cooking should be fruits (not too many sweet ones) and vegetables. Everything else, if you want to eat, you'll just have to cook it. It's a major barrier to munching. Don't buy value packs and/or keep a lot of food in the fridge just in case. You need to plan out your meals and not have things tempting you.
4. No sweetened drinks or juices. Empty calories that don't fill you up = bad.
5. No bread, rice, sugar, syrup, flour, or pasta, potatoes, except maybe small quanitites of whole grains for you to prepare, not in form of bread, because bread is too easy to munch on and eat too much. Your carbs should come from fruits and vegetables, not refined grains and sugars.
6. Don't do full blown Atkins, but watch your carbs. Atkins is a really nasty diet. I tried it, and the bowel movement issues are not worth it nor sustainable. Do watch your carbs carefully, and make sure they are worthwhile carbs like vegetables and fruits.
7. If you are fat, don't run. The impact on your joints is bad for them and often painful, and also the jiggling and running out of breath will be too much and will discourage you. Save it until you get into reasonable shape. Bike, rollerblade, or if you have access to the gym use an elliptical trainer like Precor. Those are a godsend, you can lose a lot of weight with very little impact and pace yourself and vary incline and resistance as you get stronger. But do at least 30 mins per day and closer to 1 hour. You will know it's working because there will be a pool of sweat underneath you after you are done. You can easily burn 500+ calories per session.
8. Do weights to increase your metabolism and build muscle. It helps burn fat and also makes you less jiggly and lifts your self esteem.
9. You'll lose water weight fast, and then your weight loss will slow. That is fine. Also, muscle weighs more than fat, so if you are lifting weights, your weight may really not go down even if you are getting in better shape. That is a good type of weight gain, because that muscle will burn fat too. Don't expect your initial pace of weight loss to continue indefinitely, but like I said if you do 2 hrs a day of cardio per day between biking and other exercise, and burn 1500+ calories, you will be losing weight if you simply watch your diet, not even starve yourself. Your body will burn 2000 calories per day with no exercise, so if you up it to 3500 and watch your diet, weight will go down.
10. Allow yourself some luxuries (sweets) once in a while maybe when you are at a restaurant, but in moderation. If you are burning a lot of calories per day, you can have a slice of cheesecake, just once in a couple weeks, and a small one. But don't feel like you are denying yourself things to get in shape. That way if you slip it won't discourage you from getting back on track.
