I'm going to buy a pair of shoes.
如果是给a pair of shoes提问,表示买什么,则是:What are you going to buy?你准备买什么?
如果是给buy a pair of shoes提问,表示某个行为,则是:What are you going to do?你将要做什么?
中文翻译是:我正要去买一双鞋子。
提问是:what are you doing?
what are you going to do?
What are you going to buy?