There are certainly threads regarding choosing headphones, though most of the topics in the headphones / amp / source sections are about choosing gear nevertheless. As for learing to listen to the music, it just comes from experience, you can search for topics regarding certain things you might not understand however. Also there is a page somewhere describing terminology you should know / understand when describing / understanding descriptions of sound, though I forget where it is exactly, perhaps someone could direct you to it. The one thing that's difficult about reading about headphones without trying them is that it's like trying to explain sound to a deaf person. You might get a sense of what the descriptions tell you, but what you imagine the sound of a certain pair to be can be completely different as to how you perceive it when you actually hear it. Also choosing a sound signature that fits you is very subjective and you just have to figure it out for yourself what you like / don't like. Be sure to remember what things about certain headphones you like / dislike and report them on here if you have questions. Other than that I think it's mostly experience and still try to enjoy the music lol. You can still use headphones to listen to your music, you don't have to always use music to listen to your headphones.
Final word, despite liking rap, if you want to truly experience all your gear has to offer, you'll have to move on to other music forms. Rap / pop is generally not all that well recorded, depending on the studio / editing, there are exceptions to the rule. But hearing soundstage and the range of instruments that your headphones can reproduce, you'll need other genres particularly classical, vocal, and acoustic. It's hard to listen and see if something sounds realistic if there's nothing real to begin with. Lots of modern music use synthesizers and computers to generate the sounds you're listening to, therefore you can't judge if say a piano sounds like a piano because it may be a computer. Only orchestras / live performances / overall acoustic music, can truly let you experience if the sound is true. If you could care less about this then this site might not be for you haha. If you appreciate realistic sound then I think you can still enjoy it here either way. Even rap can be better on better headphones, but you will here the imperfections in the recording and might not like everything you hear. Other forms of music like acoustic will let you hear good things in a good recording e.g. hearing the creaking of chairs in a orchestral song or the tapping of a musicians foot in others. Some of the thing you can hear will be for the better and help to immerse you, other things will be bad i.e. imperfections. It's up to you if you want to hear it all 
Sorry for the wall o' text
