Rapping is all about rhythm, so make establishing a flow the No. 1 priority when you are first learning the trade. Don't concern yourself with difficult rhymes or even making your words rhyme at all. Understand that all rappers make mistakes, so don't let it stop your rhythm if you stumble or stutter over a word -- keep going. Ignore everything else until you can keep your words flowing effortlessly.
Even if you're a skilled musician who feels comfortable playing a variety of instruments and creating signature beats, focus exclusively on rapping initially. Though background music and beats help to provide a foundation for a rap song, they are useless without the rapper. Rely on pre-recorded music loops or basic beats instead, and conserve your creative power for the rapping aspect, at least until you feel more comfortable.
Rappers must learn to use rhyming words effectively. The more complicated rhymes you create, the more impressive your rap becomes. At the start of your rap education, go through a standard dictionary, memorize a small group of words you didn't know previously and work at incorporating them into a practice song. Once you master one set of words, turn back to the dictionary and pick out new ones. Tough some dictionaries are exclusively for rappers, using these means you risk sounding like everyone else.
Unless you are destined for certain stardom, you will likely figure out pretty quickly how difficult it is to come up with words for a song. It may sound easy when you listen to a professional rap artist, but coming up with ideas is a whole other matter. Get your creative juices flowing by playing a simple beat in the background as you compose rap lyrics.