Yeah CentOS is just a gratis version of Red Hat Enterprise except with all the references to Red Hat removed (there's still some laying around), lack of professional support, and it uses the CentOS repositories (and 3rd party) instead of official RHEL ones. Though you can add those too if you really wanted.
The error you get about writing to config directories is a permissions issue. You will need to give owner and executable (x) permissions to group/user
apache (specific to CentOS, mine's www-data on a Debian server) using the chmod command. For example my output of $: ls -l is
drwxrwxr-x 2 www-data www-data /config. Then reload the server and try again. Owncloud throws some errors you can ignore, for example the integrity check can be ignored as long as you know what the files actually are. My integrity check always fails because I've put custom files inside restricted folders. In the admin panel you can generate a list of files that failed the check to make sure. This happens a lot if you start to dig in and customize the design or code. The unsecure connection warning can be ignored until you sort out https.
A self signed cert is perfectly fine for your own use or small sites, but you can run into issues in some browsers that don't like them. Chrome is one, although there's ways around it. I only use Firefox, so in that you'll have to create a security exception when you try to use https. It's really only a problem for public sites where people would freak out about the "untrusted connection" warnings. I recommend just getting regular http up and running, and testing everything with some random files before configuring it for https. Here is some excellent info on configuring and generating self-signed certs:
https://stackoverflow.com/questions/10175812/how-to-create-a-self-signed-certificate-with-openssl#
Since you're new to GNU/Linux systems this is an excellent and easy resource to help getting to grips with all the commands, navigation, and what does what:
https://linuxjourney.com/
Take a look specifically at the Command Line, Permissions, Filesystem, and Logging sections. Those will help a lot with setting things up.
What are you using to edit files? Are you just using ssh to install and configure things, or do you have a full desktop environment installed? GNOME or KDE for example. I ask because CentOS sometimes (depending on your install) only comes with vim for editing text via command line, and vim has a pretty steep learning curve in itself. I'd not recommend it for now as it can just make things more confusing. Nano is easier to use, but you might have to install it (
$: sudo yum install nano). If you're using a full graphical desktop, gedit is a nice simple text editor and comes with GNOME, in KDE KWrite is very nice. Both have syntax highlighting to help with editing the owncloud files. In nano you'd have to check the manual to see how to configure it. Highlighting isn't necessary really though unless you plan to edit it a lot like if it were for a website.
Hope this helped a bit, but I've got to run. Here are some tips for configuring a CentOS server I had bookmarked:
http://www.tecmint.com/things-to-do-after-minimal-rhel-centos-7-installation/
They actually have some tips for installing owncloud on it too.