I need To write a rap song for music and i don't know a topic it has to be a clean song anyone have any ideas?

You could write about an experience you've been through.

Base your songs on experiences you have personally gone through.
They will be more relatable, and what has more meaning than issues that every person has to overcome each and every day?
Learn how to use metaphors correctly, and expand your vocabulary as much as possible...
and remember, LINES DO NOT HAVE TO RHYME!
there are such things called "soft rhymes" as well.
just make sure your melody has rhythm, and you'll be absolutely fine

free songwriting courses by a full time songwriter: easysongwriting.net