Though I am a student doing Bachelors in a different major, I am planning to do an MS in Computer Science. Can someone please guide me how I can go about doing it? More specifically, is it common for people to change majors and do an MS in Computer Science from another major altogether?

The answer should depend on your undergraduate major. If it was fairly technical (math, statistics, physics, electrical engineering, control systems, etc.), it will be much easier to change into the CS mode. Even if your undergraduate major is not related to CS but you have developed an interest in CS via other means, there are MS (Computer Science) programmes which require students to take introductory courses as a part of their breadth requirements. These programmes might be helpful in building solid foundations before moving onto advanced courses in later semesters. Examples can be found at http://www.cc.gatech.edu/future/masters/mscs/program, http://www.cs.columbia.edu/education/ms, http://www.cs.umass.edu/grads/core-requirements, http://www.cise.ufl.edu/academics/grad/masters.shtml#core, etc. Also, if you are interested in a specific area of CS, focussed programmes might be more interesting to you than a generic MS. Examples of such programmes can be found at http://www.cs.cmu.edu/prospectivestudents/masters/index.html, http://www.cis.upenn.edu/grad/programs.shtml, http://www.eecs.berkeley.edu/Gradadm/MEngprogram.htm, http://www.cs.ucl.ac.uk/admissions, http://www.cc.gatech.edu/future/masters, http://www.cs.usc.edu/academics/masters.html, etc. Finally, it is not all that uncommon or difficult to switch majors. My masters advisor majored in Mechanical Engineering, before doing a Masters and PhD in Computer Science. There are quite a few well-known professors in Computer Science who majored in something else during their undergraduate years. CS was both my undergraduate and masters major. However, the specialization in which I took most of my courses for my masters degree was unknown to me during my undergraduate days. Though I did not technically switch majors, I did move to a totally new area within Computer Science, and I think it is quite possible for people from other majors to enter and do well in an MS (Computer Science) programme.

It's not that rare. All you need to do is find a program that's designed for outside majors - my own department used to run one and is looking at bringing it back. Wherever you go it will be quite intensive but well worth it for the job prospects. It is worth considering whether you have the inclination towards programming and/or science, first - it's no good having employable skills if you hate having to apply them.

It's definitely possible. Some universities insist that you take the Computer Science subject GRE.