I am a techie living in India. I have been invited for an on-site interview for a big tech company in London. I have never been out of India, so this is pretty confusing to me.
So my question is what type of visa is required for such purpose? Should I contact some agent to fill up the documents for me for a fee? What documents will I need from the company and what other documents do I need to submit?
You'll need a Standard Visitor visa. It is issued for 6 months and is multi-entry, so technically you'll be able to visit UK few times more. The official application portal is here: https://www.gov.uk/apply-uk-visa though the process may appear confusing to some extent. When you arrive at a consulate (or wherever is your nearest point of applying for UK visas — there's a list on that site, they will ask you to pick the least inconvenient point and time), give them the invitation letter from that company, all your docs and anything required on that site.