Dear Sheetal,
For general information about contributing to Octave, see [1, 2]
I would recommend starting with something small. For suggestions, look in the bug tracker [3] for open bugs, or look in the short projects page [4].
Once you've made a few small fixes, you could look at the projects page [5] and start on a project that interests you and that you could then use for your GSoC application.
Sincerely,
Nir
[3]
https://savannah.gnu.org/bugs/?group=octave