"This is the Resource View. The buildings in your fields all generate resources."
The buildings here produce resources for you to use in buildling, training and researching. You will initially rely heavily on the resources they produce but as you get stronger you will be able to obtain resources by attacking other players. Note that each upgrade of your resource buildings will take up idle population. This means less population available for training. Some players do not build very many resource buildings for this very reason and once you get strong enough you may consider destroying some of these in order to free up more idle population for training. Anyway, by having high level homes you can sostain a full field without worries, especially if have assigned a General who increase your idle population.
Farm are the worst building to build since they are quite useless, considering the sheer amount of food you can get by attacking anthropus camps. You should build a maximum of 1 Farm, just if you want to complete all missions, researches and achievments.
In the first years the game was out these buildings where not super useful, since stone where only used to upgrade buildings. Now every high-end troop need great quantity of stones to be trained, so having approximately 1/3 is the best choice .
These buildings produce wood. This is one resoruce you will find yourself using quite a lot so having approximately 1/3 of your slots used for Lumbermills is the best choice.
These buildings produce metal and is the other resource that is heavily utilised. You should have about even numbers of mines, lumbermills and quarries.