create username from first initial and last name

[Joshua Wahila]

I'm trying to set up a database so that a user can enter their first name, and last names, and then have it automatically assemble their username into (first initial)+(last name)+(number)

I found this string "INSERT(str,pos,len,newstr)"... and thought it might work like for what i'm trying to do

INSERT(Firstname, 2, 40, Lastname)?

I used this code to setup my database:
http://www.free2code.net/plugins/articles/read.php?id=99

so I would want to incorporate this feature into the "register" section, where it automatically assembles the username from the information the user submits, checks for duplicate entries, and if found, adds a number after it which automatically increases...

so like there would be
jsmith
jsmith2
jsmith3

someone please help?