Start here: https://cabforum.org/baseline-requirements-documents/

Those are the minimum requirements you'll have to meet to get pretty much any browser vendor or OS developer to take you seriously and consider including your cert as trusted by default. Some have further requirements.

Apple: https://www.apple.com/certificateauthority/ca_program.html

Google: https://www.chromium.org/Home/chromium-security/root-ca-poli...

Microsoft: https://technet.microsoft.com/en-us/library/cc751157.aspx

Mozilla: https://www.mozilla.org/en-US/about/governance/policies/secu...

As noted Let's Encrypt is a fairly well documented example of how this plays out in the real world.

I would of course note though that this is one of those questions where if you have to ask you probably can't do it.