Spending time listening to other major rappers, especially the ones that you admire or seek to emulate is just as important to your development as a rapper as writing and rapping is. You want to hear what they do, figure out how they do it and then practice doing that yourself.
Don’t be afraid to rap about current events, incorporate what’s going on around you and even include metaphors. Keep rapping even when you make mistakes, don’t interrupt your flow, just try to build the mistake into your rap.
Record yourself as you go even when you’re just practicing, so you can review it after and pick out great freestyle lines. Take time out to write and practice your flow every day. Practice makes perfect.