All of them are correct. For the object you can use either "who" or "whom".

You decide whom to believe. You decide whom you believe. These are both fine. "Who" is used as a subject. "Whom" is used as an object. In your sentences, "you" is the subject and "whom" is the object. Either of those two sentences I copied is correct.

While "desertjedi" is correct, especially for written English, in spoken English "whom" is not often used, and many current grammars and style guides are OK with "who." The reason for this change is that word order usually makes it clear what "who" means, and there is little chance of confusing "who" with the subject (which is nearly always first, before the verb) in a sentence like "You decide who to believe."

You decide whom to believe in You decide whom for you to believe You decide who to lay your belief at You decide who are to believe