When do you use whom? Please help.

Andy,

it's no coincidence that both 'whom' and 'him' end in the same letter. They are in what's called an oblique case. In short, "he" is like "who", and "him" is like "whom".

I doubt you ever get he and him mixed up. Well, it's the same:

He is here.
Who is here?

I know him.
I know whom?


He plays the violin.
I know Simon, who plays the violin.

Can you see how it works?

There is a longer explanation on who and whom here.