How do I transfer my SIM card from a blackberry to an iphone 4?

I'm wanting to trade my Blackberry Torch to someone for an iPhone 4, but I'm uncertain of what to do if we were to trade. Could I just put my old SIM into the iPhone 4? and I'm also confused about what would happen with the data plans. What are the risks/dangers to my phone in doing this? The reason I want to trade is that I just recently bought the Torch, but I realize I would much rather have the iPhone. Also what could happen if they iPhone I trade for is jail broken?