If you're using a laptop, there should be a key which, when pressed at the same time as the Fn key, will reduce the brightness.
If you're using a desktop with a separate monitor, there should be a button on it to reduce the brightness.
Alternatively, as has already been suggested, you can access the relevant setting via right-clicking on your desktop.
To check that your monitor is correctly set up, go here:
https://reeddesign.co.uk/testcard.htm
You need to ensure that all of the numbers, from 0 to 10, are visible on the bottom two lines, with the left hand end being pure black and the right hand end showing pure white.