This is a pirate ship with canons, It's not my canon design or product, it's made by someone else, Forgot their name....

How ever how to craft canonballs + the canon.

How to craft the canon:
Dispenser + String + Hopper (Drop them on the ground with both things)

How to craft low damage ball:
1Iron ingot 1Tnt (Drop on the ground)

How to craft high damage ball:
Take the Low damage ball and drop an tnt on it.

To use the canon:
Take your bullet/canon ball and drop it on the canon, Done! :D Simple huh? :D