[Haskell-beginners] Type Tree variables

Hello everyone, I'm trying to write a generic function which works on binary trees. For example my function works on just first three nodes of  a binary tree, but the tree is bigger, I don't care the other nodes. Like this : function Node a (Node b Subtree Subtree)  (Node c Subtree Subtree)   The problem is I couldn't find a way to represent "Subtree" structures. A guy here told me to use variables of  type Tree, but I don't know how to create them. Can you tell me how to solve this problem?  Thanks a lot.