Reading the dictionary is probably not the best option, mostly because not a lot of words in there you're going to actually use in a rap song. Fun fact for you though, and this might help you. Eminem use to read rhyming children's books to practice his flow and rhythm.
Maybe you could try that :).
No you don't have to all you have to do just listen to some instrumental rap beats and make the rhymes in your mind then just write the song.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.