Smooth Icing On A Cake?

I'm making a cake for fathers day. (Strawberry mix with either vanilla or cream cheese icing) I'm using just a regular cake icing, I'm unsure if its going to be out of the can or homemade but I wanted to know how to make a flat surface on the cake like they do at the store. I know how they do it but I don't have a flat metal spatula, and I've tried it with similar objects such as the flat side of a large knife but the cake breaks up. How can I make the surface smooth?