The first thing to try is your service provider,sometimes they will unlock the phone - for a fee , though if you haven't had the phone for long they may be unwilling to do this.
This is a newer phone and as yet I am not aware of anyone that provides codes which you can key into the phone.
I think your best bet is to go to a market stall and see if they have the equipment to unlock the phone.
A Google on Unlock Nokia 6300 shows that a few companies can unlock it , if you post the phone off to them ,and this method can cost up to �40