How To Automate Ticket Number?

I have the ff. data:

Customer Name
Date
Subject
Problem

then when the user submitted the form, the ticket number will be automatically generated with the following format:

<last 2 digits of the current year>+<last 2 digits of the current month>+<number series 001-999>

example: 0409001
(where 04 stands for 2004, 09 for the month of september, then 001 refers to the 1st record)

then all the data will be saved to the database.

my problem is how can i do this?