pronounce Zs as in THin, THrough, not like Zoo.. actually there's no voiced s (/z/) in Spanish... just an unvoiced boring s...
Pronounce C, again as in THin... if followed by the vowels "i" or "e" : Circo IPA:/θirko/ , Celebrar IPA:/θelebrar/, but if followed by any other vowel, like an English K : calor IPA:/kalor/ ...
Look at this example:
cocinar --> IPA:/koθinar/ .
you see the differencies between the first c and the second c in cocinar??
However, this may change in some South American dialects, but anyway, what I told you is what la real academia española recommends.