First, you need to download the correct iso file. Try from here:
You will then need to change directory (cd command) to the directory (AKA folder) that contains the downloaded iso.
Ubuntu is case sensitive, so downloads & Downloads are two different places.
Then run the md5 or sha256 command, for example:
Or whichever version you have downloaded.
The result will be a string of characters that should match the string for your downloaded version from this page:
If the bunch of characters don't match download the iso again.
This all assumes you have a working Ubuntu installation to do this from.