Locked iPhone, how to get it to unlock?

It's at&t and it's not my phone, it was my boyfriend's but he doesn't know how to…
but he gave it to me, so I could use it with straight talk.

Yes, his contract had ended, so he got himself the iphone 5.

Email me at stefan_zxcvb@live.com

I can help you out with the unlock

You can unlock the iPhone by approaching the network provider. Since, the contract has ended I don't think there will be a problem in getting their unlock service. Even if they deny you can approach any third party vendor like http://www.superunlockcodes.com/apple-iphone/rs2wp9/ and use their remote unlocking service to remove the network lock in your iPhone with AT&T network. Once the restriction with the particular network is removed you can use the iPhone with any GSM network.

Contact the guys over at http://www.myhtcunlock.com/contact-us/ They unlock HTC phones and iPhones, I'm very sure they will have something for you as well.