If glass or something with a glass or similar surface steams up, it becomes covered with a thin layer of water caused by steam touching it, and if you steam it up, you cause this to happen.

The bathroom mirror steamed up during my shower. 我淋浴的时候，浴室的镜子蒙上了水汽。

Going into the warm room steamed my glasses up. 走进暖和的房间，我的眼镜蒙上了一层水雾。