Open Notepad* (or SimpleText for you Macintosh** users), type any of the examples givien here (using the directory/file names on your domain), save it as "robots.txt" and upload it to the root folder.
If this isn't what you are asking, please be a little bit more specific.
* Notepad should automaticly add ".txt" to the file name upon saving, thus you will only need to save it as "robots" choosing "Text Documents" as the type.
** Users of MacOS 9 (and eariler) will need to add ".txt" to any text file they save before uploading to the web. While HTML files are still plain text, they will require either ".htm" or ".html" before being uploaded to the web.
MacOS X's text editor automaticly adds ".txt" to the file name upon saving, thus you will only need to save it as "robots".