Hi siamr,
There are a number of factors that you need to consider when applying to Business School. There are actually too many for me to name in one post, but I'll get you started with some of the "big picture" ideas, and you can define your own goals/timeline from there.
Most Business Schools have a standard set of application "rounds", which normally begin with a "first round" in October (although there are some exceptions). Round 2 is typically in December or January; Round 3 (if there is one) is normally in March/April and some Schools/Programs have a Round 4, which is later.
You need to be specific with WHY you want your MBA. Business Schools tend to prefer applicants who can explain WHY they need a specific degree, from that specific School, at that specific time. So you have to think about the specifics of YOUR situation (what you want out of the degree) and research Schools that offer programs that match your goals.
One part of the application process is the GMAT score (and it's an important part of the process, so you have to make sure that you put in the proper amount of time and effort). You can learn about what each Business School requires by visiting each School's website (although you can expect a fairly similar application from most Schools).
If possible, you'll likely find it advantageous to focus on the GMAT now and work on the other parts of the application later.
GMAT assassins aren't born, they're made,
Rich