If you aren't a machinist, prepare to open your wallet wide. If you are a machinist (or have one who owes you favors), plans are available.
There are three basic types of artillery: mortars, howitzers and cannon (also called guns.) Mortars have a high angle trajectory, usually 45 degrees or more, and very short barrels; howitzers have a medium angle trajectory and medium length barrels; and cannon have an almost horizontal trajectory and long barrels.
It helps to decide what era you are interested in (e.g., Revolutionary War, Civil War, Indian Wars, etc.) Indian Wars might fit in with cowboy shooting. The mountain howitzer was the mainstay of Indian Wars artillery fighting.
Scale is a major cost determining factor, exponential almost. Full scale pieces are the most expensive and the most impressive, smaller scale less so in proportion. Full scale generally requires several people to maneuver and trailers to move. I would not recommend full scale for a beginner.
Stay away from anything made from ordinary pipe. Pipe is an accident waiting to happen.
Do some research and ask more questions. We're here to help although you will have to make style decisions for yourself.