How to fix common errors in Nextcloud 12 Ubuntu 16.04

I get some system-warnings from Nextcloud and I have tried to use the documentation, but I can’t still fix them.

I translate the warnings because Nextcloud is on my native language.

First, the OPcache is not configured properly.
Second, no memorycache is configured.
Third, The Strict-Transport-Security is not configured to atleast X second.

I want easy step-by-step instructions because I’m new to Nextcloud