For the English doubts, the English speakers

This is not a school assignments; my classmates are far behind this level, but I must clear this doubt: In what cases must I use 'who' and 'whom'? I don't know the difference.

Please help me, and you won't have to suffer incorrect English in my posts.

As I am no english buff myself...

http://dictionary.reference.com/search?q=who

best place to figure it.

I thought it was like, depending if the first letter of the next word was a vowel or a consonant. 0_o I don't know...

^^no...

You use the word "who" as a subject and the word "whom" as an object.

A lot of it has to do with syntax.
For example
"Who is taking you?" --- See, in this sentence, Who is the subject of the sentence, just like if you said 'steve is taking you' then 'steve' would be the subject.

"Whom are you going with?" --- In this sentence, when you invert the syntax to a statement instead of question, you have 'You are going with whom.' In this case, 'whom' is the object of the preposition 'with', not the subject of the sentence, so you use 'whom' instead of 'who.'