Unfortunately there are many versions of Windows and many versions of MacOS so we cannot be completely specific in our instructions.
If you have someone with computer knowledge who can help you, it might be time to call them in.
If you are on your own, please try to make some sense of this ...
Your document has been sent to you as an attachement to an email. You need to find that email and save the attachment to your desktop (or any location where you can easily find it, for example in "My Documents" if you are using Windows).
The next step involves running the application called Adobe (Acrobat) Reader. You can do this by clicking on a shortcut to Adobe (Acrobat) Reader or clicking on the Start button, selecting Programs and then Adobe Reader.
Once Adobe Reader is running: select the File menu and then Open. Navigate to the document that you previously saved.