When you are beginning to structure a song, here are some basic ways to structure a rap song. Of course, being that rap is about being creative, you don't actually have to follow this structure. But, more often than not, this structure is what you will find in rap songs.
Rap songs usually start off with 16 bars, which usually last about a minute. After that you will have a chorus, then another 16 bars, a chorus, maybe a bridge, maybe a bridge and then the final chorus.
You will find, most often, that there are three versus and three choruses, which are introduced by a few words to lead a song in. "Hey, my name is," or I'm rapping to somebody so I'm going to intro the song to let you know what's about to happen. Then, you go into your three choruses and three versus. And that is how you structure a rap song.