I have a crush on her. 我看上她了.
Crush on her 就是对一位异性十分地着迷, 通常指的是不太熟的异性, 比如说你今天出去玩看到一个美眉长的很美丽, 你就可以用 I have a crush on her. 你可以解释成去"哈"到某人了. 这跟 fall in love with 不太一样, fall in love with somebody 多半指的是和某位异位陷入热恋, 通常你们之间已经有一定相识. 要强调非常"哈", 则可以说, I have a big crush on her.

情书范例
My honey:
I'd like to talk to you. I guess we have the same wavelength. Missing you terribly all the time, I hope you to be here at this moment. I can't live without you. You makes me feel so special. I never believe I will fall in love at the first sight until I met you. I have a big crush on you and I think I will fall in love with you pretty soon. Just be my girlfriend.