Putting Icing on a Cake

I made a cake in the shape of a rectangle, and I cut the cake into a shape. But now that I'm trying to put icing on the cake, the icing isn't sticking. When i'm putting the icing on the cake, i'm just pasting it as neatly as I can on the cake. How can I make the icing stick and stay on the cake? For the icing all i did was mix icing sugar with water. Is there a better way of making icing, that wouldn't be time consuming?