To equip an object, double-click on it or drag it to a suitable slot. Hover your mouse cursor over an object to see a tooltip with its description, stats, and comparison to any equipment currently in that slot. Some items (primarily weapons and armor) require a character to have proficiency with that item's type before they are able to equip the item.
Weapons
Armor
Ring & Necklace
Spells
Items
Pets
Scrolls
Crafting
Royal Artisans
Materials