There are maps available here if you search for the metal ore you want to farm example of gold ore http://wow.allakhazam.com/db/item.html?witem=2776;source=live
Some places are heavily farmed so it can be a real pain to get any ore in some areas but depending on your level for example Tanaris is just keep running around the outer edge and go in the silithid hives so ore there is easy to find unless another farmer gets there first.
In all areas caves are usually good places to find ore and around any hill or mountain on your maps , most areas if you are high enough level you can ride around them with a decent search pattern in a pretty fast time so its fairly easy once you understand a zone where you can find ore , even at low level its still easy once you know a zone and have spotted some ore to know where to search.