The context which Eliot mentions has to do with how important that apple is to the present moment. If someone took your apple, it means you don't have an apple. If someone has taken your apple, it means you have nothing for lunch (ie. the past affects the present). So yes, both are correct. If in doubt, start with simple past and see if that gives you all the information you need. :)

You can say either. Somebody took my apple. That is true, right? Somebody DID take it! Somebody has taken my apple. That is correct, too. You see, it all depends on the context. The story of the apple is not enough context. It all depends how the story unfolds from here. Hard, isn't it?

Both are grammatically correct and you can use either of the two in your given example / situation. You can use present perfect tense if the action results to something (Someone has taken my apple. = Result: The apple is gone.) If that's the case, then using past tense or present perfect tense is correct. However, if there is a given FINISHED TIME, you can only use PAST TENSE and not present perfect tense. Example: Someone TOOK my apple AN HOUR AGO. / Someone TOOK my apple YESTERDAY.