I am a third year undergraduate in B.E. Mechanical Engineering but I have developed a keen interest in Computer Science and I wish to do my masters in it. Will I have to appear for the GRE CS subject exam? If yes, how should I go about to prepare for it.

The GRE subject exam for computer science has now been discontinued.  All you need is to appear for GRE common exam . Try getting a score above 300 and then you can apply to different  universities. They would consider your GRE score, your undergrad GPA , statement of purpose and your letters of recommendation .I would agree with hemant aggarwal on the point that you  need  to show to the administration that you are really intrested in computer science.For that your statement of purpose will help youhttp://out.InIn addition to that you can do some research in computer science and publish some papers.    Since you will be missing on some basic understanding of computer science , you will have to take up some extra courses at your grad school. You need not worry about that as they would be recommended by the university itself. All you will have to do is work harder than the students who will be from computer science background so that you can be at par with them. If you want to persue your masters from India, then you can prepare for the GATE exam (computer science ) . Since you lack the basic knowledge , I would recommend you to join some classes for GATE preparation . Getting a rank below 200 can land you a seat in one of the 7 IITs . This is pretty good Now coming to your specific interest. From what it seems(i might be wrong) you are interested in a field of computer science called 'Human computer Interaction'(HCI). Designing of user interfaces is a part of HCI . So you can choose HCI as a specialization in your masters. For that you will have to search for schools that offer a specialization in HCI. I hope my answer will be able to help you out :)

GRE is general exam... there no exam for branch change. After exam you can apply for masters in different colleges for whatever specialization you want. But as you are from mechanical and want to go for Computer then i think its better if you take GRE CS subject exam also. Actually you have to show college administration that you are really interested in CS.