meh it comes from practice and having a witty mind. Punchline or basically jokes that rhyme. You need to be smart to be ill with punchlines. Sometimes you get smarter and your rap gets better and sometimes you get smarter from rappin. Meh just read more and develop your own style. Take shit that annoys you or take ANYTHING thats going on and make a comment about it. Only do it with irony or wit. Like I can take something random like shoes. "Everyone got air forces but no one'll join it".
Shit Like that.