I want to ftp a file to my site and make a link to it so it opens the prmompt to download and save it to their desktop. I've uploaded the file, how do I do the rest?

It depends on what the file extension is.. Browsers allow the end user to choose how they want to handle the file. If its a ZIP file or something that is not supported by a browser just placing a link to it will prompt the user to download, You can always add a "Right Click and Save As" to download