After you file

I guess my question is - what PIN do I use for the first time?  Do I have to get it from IRS.  Or I can choose my own PIN?  If I choose my own PIN, would TurboTax send it to IRS and IRS just accept that?

 

I know that IRS use last year's AGI or PIN number as safeguard to authenticate filers.  For first time filers, they don't have "last year PIN".  Their last year AGI is 0.  So it sounds like this safeguard does not exist for them?  Or I got it wrong?  Thanks!