Listen to other rappers. Every rapper sounds different and it's always good to learn from someone that has experience. If you're new to rap, listen to and study rap music and hip-hop culture for at least 30 days. Get acquainted with who you admire and who you abhor. It's during this time that you will begin to develop your rapping voice or brand.
Read copiously. The rapper's arsenal is his familiarity with language. Whether it's language from the streets or your culture, expanding your vocabulary will set you above the rest. The more words you know, the better you'll be at turning a phrase. Read any books you enjoy and add poetry as well.
Pick a theme. When you feel comfortable enough to begin writing, make sure you have something to write about. The old adage of "write what you know" is true. Make a list of all the things you want to write about.
Write conversationally. That's what rap is -- one individual telling another about a particular topic. Rappers don't rap at each other, they rap to each other. Envision yourself sitting down with someone that you want to tell something to and then proceed to tell it.
Experiment with different methods of execution. Learn techniques like pausing in key areas for effect, switching meter or using multisyllable rhyming, which is rhyming two or more times in one line. For example, "Time to rhyme, sublime prose expose the expression I chose." If you get stuck on a word, don't be afraid to use a thesaurus or rhyming dictionary to see if a word will fit and make sense. However, do this primarily to expand your vocabulary. If you plan on performing freestyle raps, which are improvisational raps on the spot, or battle raps, which are raps against other rappers, you'll need to think of rhymes on the spot.