be good at后面加什么介词

　　例句：

　　I used to be good at many things.

　　过去我擅长很多事。

　　What helps you to be good at sport?

　　是什么让你擅长运动？

　　I just want to be good at something.

　　我只是想有一项专长。

　　You must be good at learning at first.

　　你必须首先擅长学习。