To obtain a lat/lng from an address, you need to geocode it. To do that, put a TGMGeocode component, link it with the TGMMap and call Geocode method with the address.
A geocodification can return 1 or more results (until 10 with standard Google access). For example, if you geocode "Andorra" you will get 2 results, Andorra a country and Andorra a city from Spain. For this reason, the result of geocodification is an array (GeoResult). You can iterate it with Count property.
To know the lat/lng from a GeoResult, you need to read TGMGeocode[i].Geometry.Location property or TGMGeocode.GeoResult[i].Geometry.Location property (is the same)