It's really simple, eat less and exercise more.
Is the simple answer.
The key though, is motivation. If you don't have that then you won't follow it through. As soon as you can't be arsed you'll sack off the trip to the gym, or the evening jog and instead, crack open a tinnie and stick a pizza in the oven.
1. You need a goal. I can't stress enough how important it is to have a clear goal in mind. Something to aim for that you REALLY want, that will keep you motivated when you're not really in the mood.
To give you an example, when I was looking to lose weight a few years ago I used to carry a picture of myself on the beach the year before around with me. I looked horrific and as strange as it might sound, that inspired me. My goal was to not feel embarrassed when I got my top off on the beach the next year. Every time I thought about stopping at Burger King on the way home, or sacking the gym off for the evening and meeting my mates in the pub, I had a look at that picture and d'you know what? It worked. I went every time and I stuck to my plan. I dropped from a 36 waist to a 30-32 and I felt/feel so much better for it. I never put it back on either (point 2. below is the main reason for that).
2. Balanced Diet. I hate the word "diet" by the way. Mainly due to the media driven (mis)interpretation that inspires images of strange looking gunk-like shakes that can only be described as 'green stuff' and the "I only eat xxx and lost 12 stone in 20 minutes" shite arse stories.
It really is simple if you stop and think about it. Forget the crash 'diets' that might seem like a quick fix because they're not maintainable, it's as simple as that. It needs to be a lifestyle change if you want to keep it off long term. If you starve yourself for a few weeks you'll lose weight. That goes without saying. The problem being (besides it not being healthy), is as soon as you start eating as normal again you'll put it all back on. Most people know deep down what a healthy, balanced diet looks like. If you don't, do a bit or research, that's what I did.
That's not to say that you can't have a bit of what you like every now and again - everything in moderation is the best way to go. If you cut something out completely you'll eventually give in. That chocolate bar every now and then won't hurt but just not every other night. Stick in general to your lean meats (chicken, seafood, beef, eggs etc), complex carbs (wholemeal rice, pasta) and plenty of fruit and veg. Ditch the fizzy drinks and the beer (on the whole) and replace it with water.
Drink plenty throughout the day as well. Quite often when you think you're hungry, you're in fact thirsty. I have a glass of water on my desk all day, and I keep a bottle in the car as well. As for snacks, try fruit as a starter. I keep a bag of unsalted mixed nuts and raisins on my desk at work, which I dig into during the day if I get hungry.
The other thing is the times that you eat. Rather than two or three large meals during the day, try breaking it down to between five and seven smaller meals (eating every 2-3 hours). This will have an effect on your metabolism, keeping it working throughout the day and essentially, speeding it up.
Eating healthily doesn't mean eating boring either. Have a look at some basic recipes online. There's loads you can do on a budget that will be a damn site nicer than the processed shite that's available off the shelf - and it will be infinitely better for you as well. If I can cook from fresh, anyone can.
3. Exercise. What do you enjoy doing? Is there any sports that you like? As with your diet, exercise shouldn't be boring. Some people like the treadmill and enjoy going for a run. If you do then great, do that. Personally though, I don't. I find it boring as hell. As such I found a sport that I enjoy (squash in my case) and I play that two or three times a week as my main source of cardio. I never miss a session because I enjoy it, which makes it so much easier to maintain. Find something you like doing would be my main tip for this.
You can always throw in some weights as well if you wish. You don't need to be aiming to bulk up or anything like that, there are fitness benefits to weight training as well. It can also play a big part in increasing your metabolism and keeping the weight off once you lose it.
Post you're progress on here as well if it helps.