When doing a save or save as, there are two lines, the first is where you designate the name of the file, the second (defaults to Excel workbook) is the format. On this second line use the drop down until you find the CSV or comma delimited format.
Note: If you are new to saving .CSV files in Excel, you may want to play with this some. This file format can be opened w/ NotePad and might be good to look at it that way. Some of your formatting will get changed. If you have a comma in a text cell (i.e. a sentence with a comma in it) unless the text is enclosed in double quotes, when the next program opens it, it will see the comma as a cell separator. This holds true for numeric values as well. (By default, Excel should save numeric fields without comma designations).