I've seen someone (I think it was HalfAngel.) who edited this already. They stated that the correct phrase would be "an S", but they didn't explain why. I decided to help explain that ^ ^.
When the beginning sound of a word has the same sound that a vowel makes then you must write "an" before it; instead of "a".
The letter "S" has the beginning sound of "eh". Which is the same sound that the vowel "e" makes when you pronounce it. Therefore, since the letter "S" has the same beginning sound of a vowel it is more proper to write "an S" than "a S".
Other examples of this rule would be:
"The two girls bought a puppy." <-- The word "puppy" doesn't have a beginning sound that sounds the same as a vowel.
"Tommy found an apple in his backyard." <-- The beginning sound of "apple" has the same sound as the vowel "a".
I hope this information was helpful to those of you learning English.
Also, I just wanted to let you know that English is the only language I speak. Thus, I am very fluent in it. (xD If I wasn't fluent then that would be sad.)
With this said, if any of you have any questions involving English feel free to ask me!
Edit:
the English vowels by the way are: a, e, i, o, u