If you dont know where is your gears is ,then check in the Top-Right Menu you will see an icons of a Book, Bag , and Armor. As these icons are obvious just select Books if you
want to "Cast" a spell in your self . This "Cast" is your depend of your Class-Specialization. For Bag this is where you put everything you have looted from your quest. And the last the Armor is where you put your currently equiped gears. Using the Gear inventory is just simpel in most RPG game , just click the item and it will replace your current gear . To see the information of your gear just mouseOver or click the small arrow icon at the bottom then check "info".
