This page describes how to upgrade to El Capitan: https://support.apple.com/en-gb/HT206886
Before you upgrade your system make a full backup - with Time Machine or a bootable clone, so can revert to your current system, if you should not be able to download Aperture 3.6 after all. Your current version will no longer run on Yosemite or El Capitan.
2) Then I can upgrade Aperture to 3.6 as I have it in my App Store purchasing history from 2011. From what you're saying, I understand that in the App Store Aperture will change from a greyed out installed button, to a download/update button as my system will then be compatible with Aperture 3.6 to enable the upgrade.
Yes, you should be able to download Aperture 3.6, as it is in your Purchase history. But the update tab will not work. You have to move the Aperture.app from the Applications folder to the Trash (do not empty the Trash), then launch the AppStore and open the Purchases tab. Aperture should now be showing "Install". Click "install" to install the current version 3.6.
3) Then download Digital Camera RAW Compatibility Update 6.01. Presumably once upgraded this will also enable me to see the pictures from this camera in Preview as well, as I can't do this either at present? (I can only see them using the Canon RAW Software).
This RAW compatibility upgrade should be included in the El Capitan download. Aperture should be able to use it directly.
I would go first to El Capitan, see, if you can get Aperture to work well, and only then try to proceed to High Sierra. The reason I am hesitating to recommend to jump right in and upgrade all the way is, that I could not get my 2011 MacBook Pros to work well with High Sierra, but they worked well on El Capitan.
Finally, I was told that you can take your computer into an Apple Store and they will remove old operating systems from the computer which prevents the system slow downs I normally experience as I keep the OS up to date. Do you know if this is the case?
I don't know about this. You could ask this in the High Sierra forum.