Install Wordpress on Debian 6

First, I would definitely suggest looking into switching to a more modern distribution. Debian 6 was released back in early 2011 and hasn't had any updates (including security updates) in a few years. For the security alone, it's worth looking into Debian 9 (released in 2017), Ubuntu 16.04 (2016), or Ubuntu 18.04 (released just a few weeks ago). I recommend creating a new Linode with the new distribution, configuring it with your web server of choice, copying over your data, and then asking us to swap your IP. This takes some level of Linux knowledge but we have plenty of guides to help you.

If you'd like to just stick with Debian 6, you should be able to install Wordpress relatively easily - although I've never done this myself and can't guarantee software compatibility. It sounds like you already have a web server up and running. If you're running MySql and Apache, you should be able to use both our LAMP Server on Debian 6 Guide as well as the link you entered into your question, our How to Install and Configure WordPress Guide. Since Ubuntu is based on the Debian distribution, many of the commands will work for you.

Essentially, here's the process but be sure to reference the links above for specifics:

1. Make sure your webserver is configured for your domain. If not, add the domain to the webserver. Configure Virtual Hosts
2. Add a database for Wordpress in MySQL Add Database
3. Download and install Wordpress to your website's directory. Install Wordpress
4. Configure Wordpress to work with your domain and database. Configure Wordpress

If you run into anything, reply here so the community can take a look.

will it affect my other websites?

I'd suggest to test with Ubuntu 16.04 or whatever OS you'd choose before migrating all the sites to a newer OS.