I am just learning Spanish. For the most part, I find that pronunciation is very consistent. However, I am having a little difficulty with the letter "C." Most of the time it is pronounced like "K" in English, but there are exceptions. In the word "Discipulos" it appears to be silent; in "Cierto" it is pronounced like "S" in English. Are there rules for determining when C is silent, and when it is pronounced like "S" or do I just need to memorize these exceptions?
C is never silent.
'c' has three sounds.
Your problem arises with this third sound. Spanish is divided in two big areas that we can call "standard" and "seseante" (Spanish word for 's-using'). In "seseante" zones (you are learning in one of them, and they are majority through Latin America) the 'z' sound has disappeared and both 'z' and 'ce'-'ci' are pronounced as 's'. So with "discípulos": on a "seseante" zone you have an 's' followed by an 's' sound, and since there is no long 's' in Spanish you end up pronouncing just one 's'.
where z is pronounced like 'th' in 'think'
Now, depending on the zone you are, 'seseo' changes all cases of rule 2. into
I would add something to @Envite answer (I can't comment):
In spanish the sound it's more important than the way of writing (always respecting the orthografy). This means that if a word changes you have to update the way you write it.
Also, the 'z' before 'i' and 'e' it's almost forbbiden, it turns into 'c', except for very few words for ethimological reasons ("enzima", "zinc","azimut","nazi","zigzag") and in "zeta" (because is it's own name).
This is important in making plurals:
And with the "conjugación" of the verbs: