To activate your Iphone on Straight Talk cell phone service you will need to go to Straight Talk's website and get a SIM card for your Iphone and then purchase a Straight Talk Service plan.
When you get the Straight Talk Sim card you just replace the current sim card in your Iphone with the Straight Talk Sim card.
Then you just activate the Iphone on Straight Talk's website just like you would any other Straight Talk cell phone using the Straight Talk Service card.