Assuming you havent changed it from its default code to your own it will be the last 4 digits of your cell phone number.
If you manually changed this code to your own and simply cannott remember your code, you will have to take it to the dealer where they will have to send it to their repair depot (assuming they dont repair phones themselfs) and get the phone re-flashed. This will bring the phone back to factory settings and default the sec. code.
Most dealers will charge you a fee for this, usually between $10-$75.
Hope this helps.