If you want to say that you are simply in possession of something like,"I have a cat," you say, "Neko ga imasu." If you want to convey that you are currently "having" something or brought something, like, "I brought a pen," you can either say, "Pen ga arimasu," or, "Pen wo motteimasu." Imasu is for animate objects, arimasu is for inanimate. For intangible things you usually have to change the sentence around. So you wouldn't say,