MS OFFICE
I just bought MAC Book Air. I would like to know whether I need to purchase MS OFFICE software or any other any alternative is there inbuilt kind of.
Whether you need to purchase anything is up to you, however if you look in the Applications folder do you see Pages, Numbers or Keynote? If yes then think of Pages as Word, Numbers as Excel and Keynote as PowerPoint. Consider downloading them from the App Store, I think they're free?
You can open Word, Excel and PowerPoint documents you may already have in all of those programs. Even if they were created in Office for Windows.
Consider OpenOffice for Mac which is free:
http://www.openoffice.org/product/mac.html
Of course you could always purchase Office for Mac:
https://www.microsoft.com/en-gb/microsoft-365/microsoft-office