Dear Abhilash,
you find all information on the GSoC application process at
https://developers.google.com/open-source/gsoc/.
To answer your questions:
1. How and where to start learning about Octave.
Searching for "octave","tutorial"you'll find many introductions to octave, although you should already be experienced in octave if you want to apply for GSoC.
2. Whom to contact to get help.
For GSoC you need a mentor. First decide on a project that you are interested in and capable of accomplishing. Ask on this list or elsewhere for volunteers to mentor your project and convince them that you'll succeed by presenting examples of your previous work.
3. How to start developing the projects.