Remember this rule...
Prophecy (prah-feh-see)-this word has a c in it, thus making the pronunciation easy to remember (see-c).
Prophesy (prah-feh-si)-this word has a s in it. The ending is pronounced 'sigh'.
These two words are the most commonly misapplied words and it drives me crazy. Now that I've cleared it up for you, I feel better.
Learn this, and carry on!