If you already have a working Verizon SIM card, just put it inside your iPhone and start it up. It will activate with that.
If you are activating the iPhone with a different carrier (e.g. T-Mobile or AT&T), just insert their SIM card and power up. Do not power up with the Verizon SIM card installed if you do not have a Verizon account, as it will take you to a screen where you need to enter Verizon account information to activate the phone.