Definitely you do not want to decorate frozen. You can freeze the cake if you want to make it ahead of time. Make sure that the cake has cooled down before you freeze the cake. Unless you have a dedicated freezer for the cakes you do not want the cake to absorb food smells so it should be wrapped in plastic wrap and then foil paper. There is no need to freeze the cake. You can bake and let it cool completely before putting the buttercream. You will need a crusting buttercream which stiffens as you let it sit.
I make my own fondant or you can purchase the fondant. Fondant should be rolled out to 1/8" if you roll out too thin then you might get tears. Make sure that where you are working with your fondant it is cool. Fondant does not like heat. You probably need to look up some tutorials on covering cakes with fondant that will help you out a lot.
