I am very new to the German language and am wondering what the proper formal greeting would be for a cover letter to a potential employer?
Almost always correct (unless you know the name of the person you're addressing):
Sehr geehrte Damen und Herren,
hiermit bewerbe ich mich auf Ihr Stellenangebot in der Zeitung/im Internet/.... etc... etc...
Mit freundlichen Grüssen
There are subtle differences in whether to set a comma after the greeting, depending on the spelling rules in use. I can't quite say for certain which would be the current version.
Also be aware that you have to use formal language (so no "Du" or "Dein") and all occurences of "Ihr", "Ihnen" oder "Sie" meaning the recipient must be written in capital letters.