I'm finding it difficult to live with the right button clicker.
You can change this if you want so that you only have "Click" and "Control Click" (meaning you hold down the Control key whilst clicking). If you do this:
"Any Click" always = Left Click and "Control Click always = Right Click.
Method: Click on Apple Icon top left Menu Bar. From drop down, choose System Preferences.
Within System Preferences, click on Mouse (second row), then click on "Point and Click". You will see 3 options on the left.
Checking the box "Secondary Click" means that you have Right & Left Click on your Mouse.
Unchecking the box means you have "Click" and "Control Click" as mentioned above. Your choice.
I downloaded it somewhere but I can't find it.
All downloads from wherever, by default, go into your "Downloads Folder". This is found by opening Finder and you will see "Downloads" on the left. Click to open the Folder.
is it really necessary for me to have High Sierra
Absolutely NOT. Forget about it till you are comfortable with all aspects of using your Mac. Feature-wise, there is little difference; user-wise, a lot. Get used to Sierra before contemplating moving on.
Ian