I have about 9 year's experience creating/maintaining minimally secured personal or departmental web sites with Linux and Apache httpd.
Recently I've deployed a couple of web apps that require a https connection. I usually get an "untrusted connection" warning from my web browser. Two examples are public access to an OpenWrt uhttpd LuCi page over port 443, and public access to a Beaker Notebook (http://beakernotebook.com/faq), running on Ubuntu 14.04. I think the embedded webserver is nginx.
Are there general tutorials on this subject that I should be reading (as opposed to asking for "help with untrusted LuCi connection" and "help with untrusted Beaker connection?")
I know almost nothing about certificates.