You should write whatever you want to write (within reason--rips-offs of Harry Potter will more likely land you in court than on a bookshelf
). As for publishing itself, it's a long and complicated process that I don't have time to fully explain! The bloomsbury website (I think it's www.bloomsbury.com ) is pretty useful, in the Writer's section. Have a look around in the internet, there's plenty about it. Good luck if you go for it! (And grow elephant-thick skin for those early rejections!)