Familiarize yourself with different rap artists and styles. Listen to everything from old school rap like Run-D.M.C. and Public Enemy, to G-funk like Snoop Dogg and Dr. Dre, to modern rap like Kanye West and Eminem. This will expand your beat-making and rhyming vocabulary.
Develop a catchy beat. There are several ways to do this:
Use beat-making software like FL Studio or Sony Acid Pro. (See the Resource section for links.) Experiment with different configurations and rhythm patterns of kick drum, snare, cymbal, etc.
Isolate the "break" in an existing song; a break is a percussive part of a song that you can loop over and over. Many rap beats revolve around breaks from old funk and soul records.
Beatbox. Come up with a beat by using your own vocal chords and lips to simulate the sound of drums.
Use live instrumentation. Groups like the Roots and Gym Class Heroes use live drums to create beats, and you can do the same.
Write a deep and catchy bass line, or sample a bass line from an existing song that matches up with your beat.
Write lyrics for your rap song. Freestyle over your beat and bass line, coming up with a good flow and rhyme scheme to write your lyrics to. Include internal and/or external rhyme into your lyrics to give them structure. Your verse lyrics should tell a story, while your chorus should contain the overall theme of your song. For example, the verse in Grandmaster Flash and the Furious Five's "The Message" details life in the ghetto, while the chorus expresses frustrations with this lifestyle with the repeating lines "It's like a jungle sometimes, it makes me wonder how I keep from going under."
Write a catchy hook for your chorus. Many rap songs feature rapping in the verse and a melodic hook in the chorus. For example, Jay-Z rapped the verses in "Hard Knock Life (Ghetto Anthem)" and included a sample from the musical "Annie" for the chorus hook.
Arrange your rap song. Most rap songs consist of a verse-chorus pattern repeated two or three times.