ONA is an anime that is directly released onto the Internet. And from my understanding, it should be like youtube site where you can watch it however you like, with video streaming or download it. Please correct me if I'm wrong.
Where can I found ONAs legally, and what should I do to watch it, should I register or pay something?
A lot of ONAs get released either through the Bandai Channel or on Nico Nico Douga. Releases on the Bandai Channel are usually region locked and sometimes, they're re-release them on their Youtube channel (which, sometimes, is also region locked). Nico has an English version of the site which makes it a lot more accessible.
Both sites are free, Nico requires registration to view most videos and there is also a "Premium" member access.