Depending on your version of AI - File menu/Export or File menu/Save As and select pdf.
Pdf is pretty much the standard now for ads, etc. and many, if not most printers prefer it.
If you want to make an eps, it's usually best to make a copy and outline the fonts in that copy so you don't have to include any fonts with the file sent to the printer. You work off the copy so that you maintain editing ability in the original AI doc.
Using OSX 10.3.9 on a G4