Hi Aspirant

If you are pursuing ME i.e Mechanical engineering then you should appear in GATE with subject code ME.

If you are pursuing CS/CSE/IT i.e computer science and engineering /Information technology then you should appear in GATE with the subject code CS.

As you are in first year you should not be worried too much.From this time you can study your B.tech papers very seriously because the subjects in GATE exam is based on your branch papers.

The syllabus of the GATE exam can be seen by visiting the link given below:

http://gate.iitm.ac.in/Syllabus

Based on the syllabus you can choose the paper which suits you.

Hope this helps you.

All the best.