When can i use who which and that?

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Who: For use when one is referring to a person. With proper grammar, 'who' is the subject of a sentence while 'whom' is the object of the sentence. Both refer to people. In common practice though, many English speakers will use 'who' for both the subject and object and neglect the use of 'whom'.
Which and That: Both tend to be used when referring to a topic and attempting to describe it. Many people use them interchangeably but 'which' seems to add extra information to it that is not necessary to know, while 'that' adds information about the topic that specifies important information used to differentiate it from similar topics, which is a really useful thing!