First listen methodically to some of your personal favorites. Rappers have enjoyed decades of evolution through several social struggles and cultural events, all while sharing their political viewpoints and responding to one another through verse. Your own work will be richer if you can demonstrate a familiarity with the indispensable contributions of the most influential voices in the rap community. Even if you don't directly imitate a rhyming structure or tone of voice, you will be inspired and educated by listening to your idols.
Rap music is all about narrative and raw honesty. Speaking from your personal experience or profound life observations, try to write a few rhymes around a single subject to start. Whether you're talking about your experience with poverty and isolation or your disillusionment with your suburban upbringing and privilege, write about what you know. Authenticity is key when it comes to rap and hip-hop.
According to your skill level, you can write just a few lines comprising two simple verses or compose an opus of complex lyrics covering a new topic in each section of the song. Make the words flow together naturally to tell a story the listener can follow easily, no matter how quickly the words will be recited. If you plan to rap the song yourself, write based on your own abilities and limitations. Don't write complicated lines that you might fumble, and don't rely heavily on rhyming dictionaries which may make your rhymes sound forced. If the intention is to create a fun dance hit, concentrate more on a melodic chorus that is both catchy and clever. Consider having a female sing the hook.
If you have never composed a beat or instrumentation for a song, call in a professional to provide samples for the background music. A strong beat is the backbone of any good rap song. You want the music to make a memorable impact, whether it's intended for fun or educational purposes. A hard-hitting beat will sound more classic and faithful to the roots of the genre, while lush, organic instruments will give it a more modern edge. Mix in samples of old rhythm and blues or soul tracks to add another layer and pay homage to the ancestors of the art form.