It's or That's my pleasure.

「It’s my pleasure.」、または「My pleasure.」です。しかし、これを言うタイミングは、まず相手の質問に「はい」と答えた後です。

Can you get me a glass of water?

Yes. My pleasure.
Sure. My pleasure.
Certainly. My pleasure.
Of course. My pleasure.
Absolutely. My pleasure.

「My pleasure」の他にも、似たような表現があります。

Absolutely. Happy to do that.
Of course. I’ll bring that right over for you.
Sure. One moment. I’ll fetch that for you.
Sure. Would you like ice with your water?
Sure. Is there anything else you’d like with your water?