Method 1
Drag it to the desktop. Double click it.
Method 2
"Right-click" it in the email. Choose "Open With" and select an appropriate app.
Also, you were asked what format the file is, and you haven't answered. I'm going to ask an even deeper question. Do you have an app that can open whatever format the file is? For example, if the file is a .DOC file, do you have Word, or Pages, or another word processor? Or, if the file is a .XLS file, do you have Excel or Numbers, or another spreadsheet app?
Quicklook can show you the contents of a file, such as a .XLS, even if you don't have an app for it. So, if you don't have an appropriate app, you'll have to fall back on taking a screenshot of your Quicklook view and then printing the screenshot.