How to activate new sim on used iphone 4?

Alright basically i found iphone4 nearly a two years now and till this day none came to collect it so i thought i might as well keep it and the issue is when i updated everything is alright except the sim card the original sim card that was in the phone is no longer available and i think it was orange and when i put in my sim it just says invalid sim and i can't do anything.

Added (1). Thanks for your response but should i call to Orange and tell them i found it and they might take it off me or something or they can't do it?
Are they(orange because i think orange was the original sim) really able to unlock it or should i call to my sim card provider which is o2 and tell them i found this phone and want it to work on o2?

Call service provider and activate sim to that phone or they may have that phone locked to certain provider

Download a program called redsn0w on that you will find an option that will unlock the phone and activate the SIM card at the same time.

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