The letter C for example sometimes has the S sound, and sometimes has a K sound.
This site has interesting information about it.
If we look at Hebrew, the third letter of of the Aleph Bet is Gimmel, which stands for Camel.
This word was taken by the Romans and pronounced G also. To make a long story short, the Normans came along and took the letter c and pronounced it S before certain vowels and K otherwise.
So we have two ways of pronouncing C because it comes from French. So if you learn French you will know how to say the letter C.
But for the rest of us, we just have to learn it.
Here is a worksheet for practicing the letter C
Have Fun with the letter C
Rahcael Alice Orbach