Binary Tree insert the values in nodes

[andruluchko]

I have to make a model of this arithmetical expression using a binary tree. Expression is a+b*c-b
http://piccy.info/view3/7176082/d09578d27ea7057c572de3f108a9301f/
Here's the model of my tree in Java

enum NodeType { NUMBER, OPERATOR }   // Possible kinds of node.
class ExpNode {  // A node in an expression tree.
    NodeType kind;  // Which type of node is this?
    double number;  // The value in a node of type NUMBER.
    char op;        // The operator in a node of type OPERATOR.
    ExpNode left;   // Pointers to subtrees,
    ExpNode right;  //     in a node of type OPERATOR.
    ExpNode( double val ) {
          // Constructor for making a node of type NUMBER.
       kind = NodeType.NUMBER;
       number = val;
    }
    ExpNode( char op, ExpNode left, ExpNode right ) {
          // Constructor for making a node of type OPERATOR.
       kind = NodeType.OPERATOR;
       this.op = op;
       this.left = left;
       this.right = right;
    }
 }

I don't know how to implement an insert method for adding a nodes. After that I will have to print the tree in console and change nodes for making this expression (a+b)*c-b Help me please, I'm not strong in Java