Original El Capitan 10.11.0?
Assuming you "purchased" El Capitan during it's availability you can download it from the App Store in the usual way. You will actually get 10.11.6 - there are no reasons to install 10.11.0, and Apple does not have an all-versions archive.

If you did not get El Capitan while it was available then you can't get it. Sierra is the current offering, but wait a few weeks for 10.12.1.