I have a form and I want to add a \"select location\" option.
How can I do this, and how can I place a pin as the selected location?
You may want to consider using the Google Maps API, as davek already suggested.
The following example may help you getting started. All you would need to do is to change the JavaScript variable userLocation with the location chosen by your users from the drop-down field you mention.
Google Maps API Demo
The above example would render a map like the one below:

The map will not show if the Google Client-side Geocoder cannot retreive the coordinates from the address.