Listen to as much rap as possible, but make sure to find artists that appeal to your tastes and have styles that you admire. The more familiar you are with rap music, the better of a rapper you will be. It's also important to explore other musical genres. Some of the best rappers are familiar with obscure jazz and rock music and are able to rhyme over genres other than hip-hop.
Practice writing. Use a rhyming dictionary and try thinking outside of the box when you connect one rhyming sentence to another. For example, try writing a rap song like a short story that is either fictional or autobiographical. Incorporate references from the news, history, pop culture or your own personal interests.
Test your rhymes out over other peoples' beats. Find instrumentals from rap songs online or on the B-sides of vinyl singles, and rap over the music. Gain confidence in your delivery by practicing frequently. Don't try to copy or duplicate the style of other rappers; seek your own vocal sound.
Find others who rap and take the craft seriously. Once you've found people who are also working to improve as MCs, you should easily find live rap competitions. Some of these competitions consist of battling other rappers to see who can rhyme better.
Promote yourself by recording a demo and letting people hear your music. This will help to get more feedback from listeners about areas you can improve on. You should then try to meet or make friends with producers and other rappers who can help get you a recording contract.