How can I use my Yamaha i455 Indian music keyboard to play drums and piano or sitar at the same time?
You need to read the entire user's manual. You need to learn about these concepts: multitimbral operation, how to set a keyboard split point, and how to use the sequencer ("Recording") and auto-accompaniment features.
On the link you have provided in your question, at the Yamaha product information page, there are demonstration videos. Under the "Support" tab you can find instruction manuals which you can download.
I found another link to the PDF user's manual here.
There are sections in the manual with the following headings which you should read carefully:
Playing a Split Voice in the Left-Hand Area
Recording Your Performance: Track Structure of a Song
Voice Settings: Selecting a Dual Voice and Selecting a Split Voice