Rap slow at first. Develop your flow. It doesn't matter what you rap about, or even if your raps rhyme. You want to build up a cadence that you are comfortable with.
Maintain your flow. Keep rapping, even if you can't think of anything to say. By staying fluid, you eventually will get back on track. The more you stay in rhythm, the sooner words will come to mind.
Think of rhymes in advance. Once your flow has developed, you want to be able to develop actual rhymes. Don't just think of one line, but think of the following line. By putting sequences together in your head, you will become more lyrical.
Write your rhymes down. The connection between saying words and seeing words is powerful. To be a lyrical rapper, you need to make connections between words and their meanings. Metaphors and similes are good to know.
Rap about subjects you are familiar with. Fiction writers and screenwriters are advised to write about subjects they know about. The same should apply to a rapper. The everyday goings-on of your environment and current events are great topics. Popular culture also is a good reference.