There is a great tutorial on maps on a site I used when I first got started into this section of iOS. It can be found here: http://www.raywenderlich.com/21365/introduction-to-mapkit-in-ios-6-tutorial ( Just an FYI, this is a great resource for learning more about iOS. )
Anyways, this paragraph should sound familiar (as to what you're after):
You’ve now made it so that in the callout when a pin is tapped on, there will be a button on the right hand side. When this is tapped, the mapView:annotationView:calloutAccessoryControlTapped: method is called. In this method, you grab the MyLocation object that this tap refers to and then launch the Maps app by calling the openInMapsWithLaunchOptions: method.
Just note that it is opening up the Maps app, but you can choose to do whatever you want really.
Later on, here are some other great links if you need them:
http://www.raywenderlich.com - This is where I started, recommend you do the same. Use their search box. They sell some privatized/self-published PDF books too if you like that.
http://www.cocoacontrols.com - You could probably find a control someone has already written with maps.. use the search, filter for iOS only.
http://www.nshipster.com - Great building block site
http://objc.io - Great site for getting into more low-level development.