Put 何 exactly where the answer would stand and close with か. Before です and most counters it is read なん.
おしごとは なんですか。…エンジニアです。
What is your job? … I'm an engineer.
テレサちゃんは なんさいですか。…きゅうさいです。
How old is Teresa? … She is nine.
A question word does not move to the front of the sentence. 何 stands in the slot the answer will fill, and か closes the sentence: お仕事は 何ですか → お仕事は エンジニアです. The rest of the sentence is untouched.
何 has two readings. It is なん before です and before counters — 何歳 (how old), 何時 (what time), 何人 (how many people) — and なに when it stands on its own, as in 何を 買いますか (what will you buy?).
- About an adult's age, おいくつですか is the polite way to ask; 何歳ですか is neutral and normal for children.
- お仕事 takes the honorific お because it is the listener's job. About your own you say わたしの 仕事, without it.