How do you say "what” in japanese?

It's both actually. You can say 'what is it?' either as 'nan desu ka' or 'nani desu ka'. It's a matter of dialect and personal preference. 'What is your age', would normally be phrased as 'how old are you' and would probably translate as 'anata wa nansai desu ka.' I'm not exactly fluent though, so I could be wrong.