When you hear Americans spell their names,
- C is pronounced as
/'si:/, and - Z is pronounced as
/'zi:/.
To me, both sound the same. What can I do to hear a difference?
In English language classes, this issue never came up, because we were taught British English, where Z is pronounced as /ˈzɛd/.
Research I've done: Apparently, /s/ is the voiceless alveolar fricative and /z/ is the voiced alveolar fricative. I tried to find words in my native language (Austrian German) using /s/ and /z/, but, apparently, /z/ is voiceless in the southern German variants, so that doesn't help either.
Best Answer
Practise, practise, practise, and don't just listen, try to do it yourself.
Think about the difference between /f/ (as in fünf) and /v/ (as in weiß). What do you do differently when you pronounce /v/? Can you do that whilst saying /s/?
Practise.
Phonetics sites might help you. The wikipedia articles are good and contain sound samples. Personally I always liked this one but there might be others.
But anyway, if all else fails, you can always ask,