The best thing is to sit down and work out why you want to lose weight. Write a list of the reasons why you don't like the weight you are now, and the benefits of being smaller - and what you want to do at that weight that you feel you can't do now.
Then think of all the ways that will help you achieve that - break it down into diet, exercise, and psychological stuff.
With diet, you need to find your own way - whether it's going on a shake diet, following weight watchers or a similar program, following a diet book, or just trying to eat healthy and count calories - but set goals and try to stick with them. It's often easier to bring in changes slowly - so say, start eating 2 pieces of fruit a day this week, next week make sure you have 5 veg, and so on. Remember, the important thing is bringing in the changes - it doesn't have to all happen overnight, and if you flood your brain with too many new things it will rebel and not do any!
With exercise, figure out what you enjoy doing - if you look forward to it, you're more likely to do it! Try to recruit a friend or partner - this greatly increases motivation and your chances of sticking to it! Figure out if you like gym work - join a gym. If you prefer running, find an online running program with goals you can follow. You could join a pool for swimming, take up yoga, walk the dog more often, cycle to work - anything you like! Team fitness is good too - try a bootcamp or similar group fitness program, or a sport like boxing, netball, football etc. Personal trainers can be very helpful as well when you are just starting off.
The psychological stuff is the toughest part really - your head is the biggest cause of your current overweight condition, and the biggest barrier to losing weight. You need to sit down and figure out WHY you overeat/don't exercise. Do you binge eat when you are happy/sad? Do you eat when you are bored? Do you demonise food? Some people like to visit a counsellor or psychologist or GP to work these things out, or you can do it at home - there are lots of websites that can help you with "emotional eating" and various other issues.
Hope that's a decent guide of where to start!
Finally - this is all up to you. Others can motivate and help, but ultimately you are in control of your own destiny - whether you lose weight, whether you keep it off, whether you quit tomorrow - it's all you. You need a good driving force (that's why I said at the start - work out WHY you are doing this) to keep you going.
And remember, if you fall off the wagon, just jump right back on! Up and downs are what make us human, it's life - no-one does this without a few hiccups along the way! If you never quit, you can't lose.