How can I locate my stolen phone with find my iphone?

These are the steps:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

or

Use the Find My app to locate a missing device or item - Apple Support

Regards

Giulio

Go to the 'Find My iPhone' section at https://www.icloud.com. 'Find my iPhone' has to have been enabled on the device before it went missing, and it has to be on, not wiped, and able to connect to a network.

In the event you are able to locate it you should not try to retrieve it yourself: that could be dangerous.