Hi Ankur,
I am doing youtube marketing from last 7 years and completed more than 20k+ customers for my first product so most of the sales i made from youtube.
I don't remember (it was too long time ago), but probably SEO :D
In my latest businesses, the first customers come from Twitter in most all of the cases
DM on Facebook.
Typically you get first customers from a network or existing relationships. Then you get referrals, and then you start selling to strangers. That's when it gets harder because people don't know you or your product.