Warm up with some gentle arm stretches. Use some of the warm-up movements you use at the gym before your workouts. Do this for five to 10 minutes before developing and rehearsing more elaborate movements. Being relaxed during this process will help your creativity to flow.
Act out the story in the lyrics during your rehearsals to find gestures and hand positions that tell the story in your songs and raps. Simply allow yourself to flow through the verses without judging the gestures that you discover. Choose movements that you like, and run through your songs a few times until your favorite gestures start to feel like second nature.
Watch performances of some of the artists that inspired you to get into rap. Learn from them and grow your arm movement repertoire. Limit violent gestures and those that may incite a negative reaction from your audience. Do not imitate any particular rap artists, but allow what they do to inspire you to find movements that are true to you and your music.
Hire a choreographer to help you develop movements to go with the story in your songs. A choreographer can help you discover gestures that you may not be able to come up with on your own and give you more variety to play with on your own time. If paying a choreographer is out of your budget, contact a local arts college or dance school to see if any students or instructors may be willing to assist you for free for an hour or two.