You might want to start out with a nice little golf ball mortar similar to what Double D has in his avatar there.
Unless you have access to a fairly large lathe and tooling to make a deep hole, a long cannon barrel with a golf ball bore might be difficult to make. However, you can make one from seamless tubing and plug the end if you use a safe design and have the equipment and ability to do so.
You can also buy a barrel already made, then concentrate on making a mount for it. Many mounts for small - medium bore cannons/mortars can be made without heavy equipment or materials.
When making from scratch, it all depends on what your skills are, what equipment/materials you have available and how much time you are willing to invest.
I've made cannons/mortars from scratch, bought barrels and made mounts for them, and bought ready-made guns or kits. In recent years I've have more money than time, so I buy more than I build. When I retire (Lord willing and the creek don't rise), it will probably be the other way 'round.
If you decide to buy a barrel, please take a look at the vendors here. There are some makers that offer very good barrels at reasonable prices.
I bought this small mortar recently, and it's just as much fun as any larger cannon or mortar I've ever had. No hernia required to get it into action either...