Iphone 4s lost sim card to activate what next?

Well, my father got me the Iphone 4s on contract like a week after is release but I was in a foreign country at the time studying for two years. Him trying to do the right thing did not open the Iphone box to insert the simcard to activate the phone. He bought an unlocked phone from carephone warehouse only to be informed that when I got the phone without its activation simcard I still was unable to use it due to two things is neither activated nor unlocked.

The problem lies with the fact that I don't have the original sim to activate it as my father got the contract but found that that it was useless paying a contract that goes daughter was unable to use so he cancelled it. Thus original simcard.

What can I do now to use my phone now that I'm back? Will a simcard from its operator be enough to activate it? If not then what?

I'm pretty sure any sim can unlock that phone, but it has to be according to the same network the phone was bought for, eg. O2