In Darkfall there are several different types of cannons, ranging from fixed positions on ships, on warhulks, and on clan city towers,  to deployable cannons that you spawn with a figurine in your backpack.
To deploy a cannon from a figurine in your backpack, aim at the ground where you want to place the cannon and double click the figurine in your backpack. Once a deployable cannon has been placed, it can never be un-summoned, so take great care when placing it.

To use a cannon, walk up close to it while aiming at it, and press F to use it. Once you have control of it, you aim it by using your mouse, and fire by left-clicking your mouse.