Swapnil said:
I don't know much about M-theory except for some basic laymen concepts, but I was wondering if one were planning to study M-theory, does he/she needs to have a strong understanding of QM?
The mere fact that you are stating the question in this fashion suggests that you still have a long way to go before doing anything like what you're asking. Still, if you must know...
Learn quantum mechanics. Not the average "Griffiths introduction to" type. There are many good books out there, so I'm not going to give you a list.
Learn quantum field theory. Zee is indeed a good book, if somewhat sketchy at times (this is not a criticism of the book, it's meant to be that way). Zee will help you tackle the lengthier tomes (Peskin & Schröder, Weinberg, Kaku). Mandl & Shaw is also a good intro book.
Learn general relativity. Weinberg is still a great book, but Sean Carroll's online notes are excellent as well, and contain a nice deal of mathematics.
Speaking of which...you cannot learn enough math. Algebra (representations in particular), differential geometry, topology, algebraic geometry... there's a lot out there.
A final word, there is no such thing as M-theory, not yet anyway. We have only a vague idea of what it should look like.