You can follow the instructions here to get to the files area and upload a file.
You can place it anywherein the files system - but a custom folder may work best. If it's a link you plan on using for your public site, make sure to keep it in the public files area.
If you want to place a link for downloading, you'll need to upload the file, and then copy it's location. the easiest way to do this (once it's uploaded) is to right click the file and tell it to open in a new window. int he new window, copy the url. That is the file location.
In the body text area of your piece ofn content, create a hyperlink (using the link button - it looks like a word with a few chain links in fornt of it), and then link it to the attachment's URL.