The rap song is a modern form of poetry and it is therefore the quality of the lyrics that distinguish the good from the great rappers. A great rapper will create a song where the lyrics are personal and where they flow very easily. The words blend into the music but they still manage to make a good point. Being able to create a great rap song will take a lot of practice, and the best way to begin is to simply get started with a pen and a piece of paper to see what you can put together.
The first step in creating a meaningful rap song is to decide on a theme or topic. This could be about an event that has happened, an issue that is worrying you or that you want to draw attention to. It could be a dance song or one that is mainly about yourself or even something that you had a dream about. The truth is that it does not matter what you choose to write about because there are no wrong themes. And at this stage you do not have to worry about a title, that can always be decided later.
While your lyrics need a story, you are not required to tell an actual story. However, there are some rappers that are most successful when they tell a story. Just make sure that the song is complete with a start, a middle and an ending. The listener needs to go on a journey. One technique used by some rappers is to create the song in paragraph form first, then use this to formulate the lyrics.
Amongst all of this is the beat. You should not opt for a beat that you are not comfortable with. If you are just starting out, for example, you might not be able to rap very fast so a fast beat will not help you. Listen to the beat a few times to figure out if it is what you need.