The most you should expect to loose is 1kg per week, that's if you're really good at sticking to your plan. In the first week, you might loose 2 or even 3, cos your body gets really excited - it's not used to functioning with less! And you might loose extra if you get sick, but don't plan on that. It's not fun. So I would say the least amount of time would be about 4 months, but the slower you loose the more likely it'll stay off, so don't say it has to take that long.
With plans and stuff, you need to think about what will work for you. I don't have the time or the money to go to meetings. But some people find that environment really stimulating. Also there's the incentive that you're paying for it, so you'd better make the most of it, and the group weigh-ins, well, you don't wanna be embarrassed, so you might try harder. There are shake plans and stuff, but would you get bored with only eating milkshakes? Would it suit your family? Counting works well for some people - you take the time to write down everything you ate, assign everything a number (calories, or points with the WW plan), and only eat a certain number each day. I generally can't be bothered to though - I forget. Would you? There's a website called calorieking.com.au that a lot of people here use - it's good stuff, makes it easy, plus it's free. If that will work for you, that is.
Personally, how I work is by knowing stuff about what I eat and what I need. You can learn it through magazines, books, the internet, packets, etc (though don't automatically trust everything you read, even if Dr So-and-So says it, especially if they're trying to sell you something). Pretty quickly, you get a feel for the right and wrong foods to be eating, and the amounts. Some people have trouble with this, or like to be more precise, that's why they use counting methods and such.
You just have to do some research and think about what's going to work for you. And there's no reason you can't try a few different things before you decide. Shop around.