No i haven't bought a Vauxhall

Built a garden shed out of some wood and sheets i had given. At the moment it's got 20mm thick plywood round it and i need to finish the final appearance off. Can i just use ordinary tongue and groove boards and creosote them or use treated wood for it to last ? It's about 2.4m square and i don't wanna spend a fortune on treated stuff if i don't have to. What alteratives are there also ? Any ideas/hints welcome