You still have a long way to go since you are still only 18 years old. You can think of going for an mba only after another 3-4 years when you get a bachelors degree. DO NOT go for an online mba. They are of no use to you if you aren't strictly disciplined. If you do not get into any of the IIM's, it is not the end of the world. There are so many other institutes in India you can apply for. Though IIM's have a brand name, it will all be up to you to get noticed.
Do not worry, your time will come