When speaking Spanish, how do you pronounce the letters "c" and "z"?

[solarstorm]

I depends on where you want to speak Spanish.
I Mexico you pronounce caza and casa, e.g., the same way: like an s.
In Spain you have to pronounce the z in caza similar to a th. The s in casa, however, is to be pronounced like an s.
Edit:

I didn't read your question properly. The pronunciation of the c also depends on the following vowel.