How to activate my new iPhone 4?

So my best friend gottm the iPhone 4 for Christmas and I was really excited but now I'm not sure if I can even use it. My phone carrier is Straight Talk and will stay Straight Talk. I don't plan on changing. The problem is, the iPhone he got me is from Verizon. I know that Straight Talk has iPhones, but to use them and activate them through Straight Talk, you have to either buy them on their website or at Walmart through their carrier plan. I'm not sure whether to tell it is an unlocked iPhone (meaning you can put any sim card in from any carrier and have it work) so if it's not unlocked, does that mean it's useless? I would hate to have that much money go down the drain because he spent a lot of money and I'm so grateful for my best friend.
What do I do?

I'm sure there's atleast one sim-card which fits the iPhone 4. Figure out which one it is and try activating it, else you have to visit the apple store (Genius Bar). Check the additional information.