In exactly the same way that any other boy would ask for a deodorant. If you are asking, "How do I tell my mother or father that I need a deodorant?" Just tell them that you think that your body isn't smelling as fresh as it should be over the course of a hot day at school or something similar. If you mean, "How do I ask for a deodorant in a shop?" Well if you go to a supermarket or an large store you usually just pick up the items that you want and then take them to a checkout, so there is no need to ask anyone. In a smaller shop it might be advisable to ask for a deodorant by brand name, such as Lynx, Sure, Life Guard etc.