I am showing the current location in MKMapView by using showUserLocation enables. I also want to center the mapview to the us
For centering on user location, you can use the following code:
[mapView setCenterCoordinate:mapView.userLocation.location.coordinate animated:YES];
For zooming on special locations, you should study how the regions (MKCoordinateRegion) are counted and work, count your values for the region and display it using call:
[mapView setRegion:myRegion animated:YES];
This sample WorldCities shows basics of region displaying.