How do I minimize the loot items when being attacked by other users?

Don't ever try to "save" resources in warehouses (top of screen)!

Best: save them in your bag as items (no attacker has ever access).

Or save it in your storages AND disconnect ALL storages from ALL roads

(attacker won't then have any access as well)!

Use resources to fill your warehouses olnly if you are going IMMEDIATELY

to upgrade/research something (and then put shield before doing upgrades/researches).

Unless you use constantly shields...

To increase enemy losses yo must make yourself stronger. This is done with heroes skills, more troops and research for your Legions, and troops.

When the enemy wins they will raid your resources depending on how much load they can carry, what is available in the stores at the top of your screen and any depot protection that you have.

Try to keep the stores low by using up the available resources for upgrades / research unless you are saving up for something. Farmers will have a good level of depot protection by default. If you are not a farmer then you will rely on alliance technology research to give you a small level of depot protection.