Remember it is your health which you are after, not just your weight.
Diet plans, buying magazines, fads, gym membership, personal trainers etc etc are, by themselves, a waste of time.
As others say, consistency, consistency, consistency.
As you do more exercise, you will naturally need to take in more calories, that is OK so long as calories you burn is greater than calories you eat.
Sooner you start the better, and then keep it up all your life (that's what I try to do anyway
). Make it part of your lifestyle. Treat it like work - do not do your workout only if there is a very good reason not to, and 'I don't have time' or 'I am tired' is not good enough reason. Only good reasons are 'I have flu', 'I am injured', 'I have a family emergency'. Even if you only have time for 30 mins workout instead of normal 1 hr, do it.
Breaking into the new habit is the hard part, start today, not tomorrow. Minimum 3 days a week exercise. Start slowly - walking 3 miles a day is a good start, then after a week of that, jog for a hundred yards and walk the next 300 yards etc. Then gradually the next week do more jogging than walking. If you build up slowly you will find it comfortable to achieve.
I run around Memorial park a lot, very inspuring to see so many others exercising, keeps motivation and costs nothing.
I also laugh at the joggers/walkers who don't break into any kind of sweat or exertion, and seem to spend more energy talking to their companion than doing exercise. That is just playing at fitness.
What about entering a 5k charity race a few months from now? Good motivation.
I concentrate on running, which is excellent for fitness and weight, but you may want to mix other exercise - bike riding, walking, swimming some gym cardio.
I can;t stand gyms and treadmills and stuff - I reserve getting sweaty with other people to bedroom activities.
Here's a thought, combine exercise and pleasure, lie you partner on the bed, tell him not to move, and fuck the shit out of him for 45 mins, then take a glass of water. That should work, if repeated every other day, but you may have lower back problems.
My weakness - alcohol. Weight loss is difficult if you like a few glasses of wine a day.
There are plenty of debates on stamina v speed/cardio work, main thing is to mix them, but there is some evidence speed/cardio work is more efficient at reducing weight becuase the metabolism is raised for a period AFTER the exercise, so weight is still coming off e.g. while you are asleep. But building stamina is good too.
Enough rambling...don;t believe everything you hear from those trying to sell you something, you can get fit and reduce weight for free if you want to.