Hi Siva,
Sorry about the delayed response.
We have a basic workflow guide on github,
https://github.com/mathics/Mathics/wiki/Contributing.
The usual recommended starting point is to take a function you use/know from Mathematica and see if it's implemented correctly in Mathics.
If not, have a go at implementing it and submit a pull request. Even if your attempt isn't quite correct someone can give you some pointers.
Let me know if you have questions :)
Angus