Hey Snehit, it sounds like you are interested in transcribing. Which means, figuring out music by ear and putting it on paper, i.e. notating it. To do this, there are few areas you will need to develop:
1) General understanding of harmony. Start with our music theory course, it will provide you with the fundamentals.
2) General understanding of rhythm. The best way to understand how rhythm and timing connect is to learn to read standard notation. Start with our course on reading, but after that you will need to begin to learn to notate. And for this, I’d suggest picking up a copy of this book.
3) Lastly, you’ll want to start developing your ear. The best ear training tools can be found here. Start with the interval ear trainer (bottom of page).
4) I’d also recommend picking up a slow-down program to aid in transcribing. This is the one I use:
Here’s the download link for Windows.
Here’s the download link for Mac.