How to activate imessage on my iphone 4?

I bought an iPhone 4 from my friend and he turned off the service and I was wondering how to turn on imessage. I tried but it just says waiting for activation then in like an hour it says activation unsuccessfull.

Here's what I did

Get a SIM card that you or no one else uses. If the phone is AT&T get a paper clip and stick the end in the hole in the side of the phone. Place the SIM card in, then close the SIM card tray. It will say no service but if you are in Wi-Fi go to messages and log into your iTunes account. There ya go.

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