if it's just on the surface and your wall is white, you can use sand paper or sand block to sand the mark down.
if the scratch is deep, you will need to sand it, patch it with wall paste and prime and paint that area.

谢谢。我想应该就是在表面。可是在哪儿能买到砂纸呢

Use Magic eraser. Works great for many types of small stain. Only 2 dollars for small pack.

how about chalk?

是什么样的eraser?在哪儿可以买到

用了粉笔可是又象在墙上画了一条白线。用手一擦粉笔灰就会掉，污迹还在那儿

不是经常在电视上有广告的嘛？一块象海绵似的东西，什么都能擦掉，越用越小，最后扔掉就行了。应该在卖家庭清洁用品那儿。

You can get sand paper/block at homedepot.