Try to use the guide for Debian 7. Much should be the same! If you run into specific error you can ask here 
@Jenkler Okay thanks. I didn't know what to do since it says "ONLY Debian 7" and I'm kind of new to all this. So I just followed the documenation until the "clone this repository" step (which did not work, because there is no such directory ("or file"). Here the errors from the other steps:
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LANGUAGE = (unset),
LC_ALL = (unset),
LC_CTYPE = "UTF-8",
LANG = "de_DE.UTF-8"
are supported and installed on your system.
perl: warning: Falling back to a fallback locale ("de_DE.UTF-8").
locale: Cannot set LC_CTYPE to default locale: No such file or directory
dpkg: error processing package redis-server (--configure):
subprocess installed post-installation script returned error exit status 1
Processing triggers for libc-bin (2.19-18+deb8u4) ...
Processing triggers for libgdk-pixbuf2.0-0:amd64 (2.31.1-2+deb8u5) ...
Processing triggers for systemd (215-17+deb8u4) ...
Errors were encountered while processing:
redis-server
E: Sub-process /usr/bin/dpkg returned an error code (1)
cd /path/to/nodebb/install/location
-bash: cd: /path/to/nodebb/install/location: file or directory not found
I hope someone can help me here. Thank you very much! I'd love to run nodebb!
You're actually supposed to create a directory. Assuming you are a regular user and don't need multiple nodebbs, the home directory should work fine. (that's what i use)
cd ~
git clone -b v1.x.x https://github.com/NodeBB/NodeBB.git nodebb
I'm not really sure, never seen those errors. But it seems to only have failed on a post-installation script for redis-server. So it might still work.
tbh, you probably want to remove the redis-server package and install it from source. This is the method I see recommended the most.
http://redis.io/topics/quickstart
@yariplus You're right, it's running now although I got these errors.
However, I still have to use port 4567 instead of a sheer domain name. Tried to follow this tutorial: https://docs.nodebb.org/en/latest/configuring/proxies/nginx.html
Nginx in my case was installed through plesk and I can't find the config file nor do I know about possible consequences/effects on my other sites hosted on the same server via plesk. Any suggestion on how to proceed to be able to remove the port? Thanks!
If you use NGinx for the other websites that you host, and when you want to install it to a subdomain which is not being used now, there will not be no problems using that tutorial. Make a new file (so do not use the default setup, or an alias that is already there) at /etc/nginx/sites-available and a softlink to /etc/nginx/sites-enabled
touch /etc/nginx/sites-available/nodebbforum
ln -s /etc/nginx/sites-available/example.com /etc/nginx/sites-enabled/nodebbforum
You can use the setup of the tutorial then by changing the file /etc/nginx/sites-available/nodebbforum. Make sure that server_name is set to the correct subdomain.
@JasperNL There are no such directories as "sites-available" or "sites-enabled" there. I forgot to mention: Plesk installed the newest version of NGinx: 1.11.1.
By the way, me being relatively new to this kind of work: How come "ls -al" is not showing me the "etc" directory in the root although I can access it via "cd"?
Thank you very much for your help! 
@Hillcow Sites-available and sites-enabed does only exist if you install the nginx from the OS repository if you install from any other repository you will only have conf.d which is a good place to put your configuration with the small change that the files in there need to end with .conf extensions.
this should work for you, you should've mentioned you are using plesk on the server early on
since it completely changes how apache2/nginx configuration works
https://community.nodebb.org/topic/9215/running-nodebb-with-plesk-tips
