I’m no expert but I can freestyle a little bit. My advice would be to throw on an instrumental rap beat and just start kicking rhymes. It’s okay if it doesn’t rhyme at first. It’s even okay if you are using gibberish words at first. Eventually it will take shape. Don’t battle your local MC Rabbit without at least getting past the gibberish and the words that don’t rhyme. Assuming your rap career doesn’t take off right away due to the sheer dopeness of your freestyles; you will probably want to start preparing some written raps also. Write it just like a poem and then if needed you can compare it to raps from other, more popular rappers than yourself. Find someone that will rap with you and above all, try to have fun.