SIM cards and activating phones?

[AC Question]

I just bought a samsung galaxy s3 and need to activate it with Bell. It a Virgin Mobile phone but I was told by the clerk and Bell that these phones can be activated interchangeably. Does the SIM card have to be inserted in the phone and does the phone have to be turned on before it can be activated?

 

[RumoredNow]

How do I activate my mobile phone or my SIM card?