How to generate automatic bill number from database in c#

Question

 Hai,
 I have one doubt.I am doing billing software. In this project the bill number want to generate automatically from database and also it should be unique like bill_01,bill_02....., and I want to display it to one textbox. After that I want to saved all details to database.
Please help me for how to generate bill number automatically from form loading.....
 Thanks in advance