If the letter “C” is followed by the letters “E”, “I”, or “Y”, pronounce it as an “S” sound.
This is the case in words like “cellar, city ,cyst”.
If the letter “C” is followed by the letters “A” “O”, “U” or a consonant, pronounce it as a “K” sound.
This is the case in words like “cab, cola, cut, acquaint”.
Even with this rule, please keep in mind the most consistent rule in English, there is always an exception to the rule.