I think different people fantasize about different things. If you have a crush on a friend, it seems to me that it would naturally follow that you might masturbate to the fantasy of you and that person having sex. Likewise, I don’t think you can easily define what “you should and shouldn’t be aroused by”, in the sense that your fantasies are what they are (and are unique and personal). That said, society has created a code where certain things are frowned upon not only in practice but also in fantasies (like incest or pedophilia). However, to say that no one ever has incest or rape fantasies (for example) is ludicrous.
Judging by your recent string of questions, it seems you are experiencing an awakening of sexual interests. I hope you take your time to learn how to be sexually active in a healthy and responsible way.