1. One of the most important things at this stage is for you to map out what career path you're interested in, and start doing some research and networking. It will come in handy in your essays and interviews to have specific knowledge of what your goal is and how you will achieve it.
2. You can also start researching schools. Figure out where you can get in, where you will fit in, narrow down your list and learn a lot of details about each school and all of its academic and extracurricular programs. Getting a head start on this will give you a huge leg up on applications and there's no reason you can't do most of the work way in advance.
3. Once you've spent some time on 1 and 2, start thinking about big picture questions like why MBA, what are your career goals, why school X, what are your strengths and weaknesses, and other things that are commonly asked as part of the application process. Start taking notes that you can eventually work into an essay.
On extracurriculars, think of it this way- Your advantage vs. other candidates is leadership experience, your disadvantage is business experience. Keep that in mind as you try to build a balanced profile. Extracurriculars can be an opportunity for you to demonstrate some interest in your post-MBA career field- for example if you like marketing, you could help a local charity run a fundraising campaign. If you like finance you could start an investing club. Doing something relevant is more important for you than getting more leadership or community service experience in my opinion. Obviously the more you can do the better, but you have to be judicious with your time.
You have the time, so budget 6 full months to study for the GMAT. No need to rush it.
This isn't a step-by-step five year plan, as in complete task 1, then task 2 and so on. You'll have to continuously work and develop each of these things until application time. But once you begin applications, it helps to be very organized and structured.