How to Rap. It's been said that rap is the biggest pillar holding up the poetic pedigree in this country. Metaphor, rhyme, punning and imagery of all varieties are common to both art forms, and it doesn't take much of a stretch to consider them both simply "verse." Rap, though, is generally performed with a musical slant, and takes several cues from hip hop and popular music as well.
Just start "flowing." Rap is largely improvisational, so just start speaking in the vein of free association. It doesn't matter what you're saying, just let the words come forward to get a feel for the medium and keep going.
As you become more adept, work traditional poetic techniques into your rap. Use metaphors to compare things and rhyme to give your verse a musical quality. Puns and wordplay make your rap clever, but don't forget it has to sound good, too. Remember: it's music.
Practice, practice, practice. Rap as you walk or work. Always be rapping about what you're doing and thinking. Write down clever rhymes or wordplays that occur to you during the day. Keep building on the foundation you laid.
Write a rap. Although rap started as improv, there's no law that says you can't write it down ahead of time, and doing so will allow for a much more intricate and well-layered lyric.
Have a rap battle. The ultimate test of your rapping abilities, and one of the fastest ways to hone your skills, is in competition with another rapper. Insults, clever jibes and lightning quick thinking are all elements in the rap battle, so practice with somebody of about your ability.
Don't worry about looking or sounding good at the beginning. Just worry about making progress. Practice in the back of your mind as much as you can.