My iphone got stolem, and i have Findmyiphone app but what do I do when I locate it?

My phone and credit card where just stolen last night and I dint think about the "Find my Iphone" app until 3-4 hours later but by then it was offline, I know someone stole it because when i called it they returned my house call but only let it ring once,
So what should I do when I locate it next time it goes online? Should I call the cops to come with me? Or what can I do to get it?
I'm thinking about going with some friends and forcing them to give it back but at the same time why do that if i can just take a cop but then again I know you can't search someone's house without a search warrant… But could they search the person if its in the streets?

Firstly, put it in lost mode and, if you want, erase it. Then they can't steal your stuff on the phone. That is the most important thing. I would never go after them myself, but the police will probably be useless. You can try that if you want, but a better idea would be to contact apple and, if you have apple care, get a new phone.