How do I maximise resources plundered, when I attack my farm I only get food?

Resources plundered will depend on 2 things, how much you can take and how much the enemy has 'above' any depot protection level. If you are attacking a farmer account then they will have a high protection level.

The relative levels of the enemies resources will affect the mix of what you get. If you attack an enemy with lots of food and little of anything else then the vast majority of what you take will be food.

To maximise what you take upgrade the load capacity by having more troops and using only cavalry as they can carry more. Then try to find targets that are not farmers and have a reasonable amount of the resources you want. Scouting will tell you this although its not always possible or a good idea to scout first.

I was told to switch my farmer to trader on my farm accounts. I saw a HUGE difference in the amount of resources I pulled.