NOTE: Many people claim that ‘whom’ is no longer relevant. While ‘whom’ is certainly on the decline, it is far from dead. You will undoubtedly run into teachers who support the use of ‘whom’ (but who probably doubt that anyone can properly use it); you can impress him or her with your grammar prowess.
If this is still confusing to you, rephrase your sentence to avoid the entire ‘who’/’whom’ problem.
If you have to take a wild guess between ‘who’ and ‘whom,’ choose ‘who.’ (We use it much more frequently, and very few people expect you to know how to use ‘whom’ anyway.)