Re: where should i get this TAN ?
Welcome to AdWords Community.
Google does not offer TAN, it is offered by Income Tax Department of India. You can find the application process at this URL https://tin.tin.nsdl.com/tan/form49B.html
Additionally, if you do not have a TAN number and you are an Individual not a company, you can leave the TAN field blank while setting up your AdWords account. TAN is required only for business entities, not for individuals.