Binary Tree: Change from a single character to a string

i have this code and currently you can enter and uppercase letter and it will enter it into the binary tree and sort it alphabetically, but i need to change it so that you can add a string of say 20 characters and it will sort it by the first letter of the string only, and at then end where you type > then enter, it will display all the strings in alphabetical order ??

thanks for anyone who can help