Congratulations! I'll answer your questions in order.
1. Most top international business schools do require a minimum amount of full-time work experience - typically 2 years. The average # of years of work experience for admitted students is usually in the 4 to 6 year range.
2. If you ask that question here, you are going to get "my company" as the answer. :) I encourage you to research multiple companies to see which one you think would be best for you. We have both interactive self-study options (you are studying on your own / not with a live teacher, but you have a series of interactive lessons that adapt to your skill level / needs) and live study options (you take a class that meets in real time).
We have in person classes in the US, Canada, and London; we also have online classes. I have people from all over the world in my online classes. We let people sit in on the first class session for free so that you can see whether you like it. If so, and if that class hasn't already sold out, you can join the class. (If it does sell out, you can join a different class.)
This article might help you decide how you want to study:
https://www.manhattangmat.com/blog/inde ... or-part-1/3. Most people study for about 3 to 4 months, though it obviously varies from person to person. Unless you don't need very much of a score increase, I recommend 2 months as the minimum. Ideally, you wouldn't want to stretch your study out longer than 6 months, or you may find that you are just forgetting what you studied.
4. GMAT scores are good for 5 years from the date of your test, so if you take the GMAT in, say, November of 2014, your scores will be good until November of 2019 - plenty of time. :)
5. Most people do work full-time and study simultaneously. If you know that your job is going to be very demanding (some people work 60+ hours a week) and you have the flexibility to complete your GMAT studies first, then you might want to consider taking the GMAT before you start working. But, as I said, most people do work at the same time. Just know that you're going to be very busy for a few months!
6. Business schools use a variety of information to make their decisions, including GPA, GMAT, personal essays, work experience, recommendations from others, and so on. I would recommend taking a look at the current applications for several schools that interest you so that you can see the kinds of things they ask you to provide.
In sum, it sounds like there's a very good reason for you to start work in September, so go for it - your work experience is one of the most important parts of your b-school application. Give yourself a month or two to settle into the job and get the rhythm of things, then go ahead and start your GMAT studies.
If you decide to take a class, I'd recommend signing up a few weeks early so that you can do some pre-work before class starts. In our program, for example, we include two Foundations books, one for quant and one for verbal, but we assume you already know that material when class starts. If you don't, it's better to learn before the program starts than during the program.