I forgot my phone security code of nokia 3315 cell

When you place a new or a existing SIM card from another phone provider into your mobile phone and it asks for a Security code then :-
This phone is locked to the original mobile phone provider. You need to contact this mobile provider to ask for the unlock code (security code) for this phone. You need to provide proof of ownership etc and the phone's unique IMEI number which is located inside the phone under the battery. They use this number to generate the unlock code for your phone.
They should provide you with instructions on how to enter this code into the phone.
Once you unlock the phone, then you can insert a new SIM card in this phone or sign up with another phone provider using this phone.