To begin with understanding the User-Interface and properly using it, you must first understand that it is divided into 5 different sections.
This area shows the environment around you which consists of terrain, hostile enemies, friendly allies, and NPC's.
Each slot in this section is where you will equipt your gear through out the game. This includes weapons, armour and even jewelery.
This is the area where all players and NPC's can communicate through many channels. The vast majority of channels in the chat are public but some, such as clans and clubs are private.
This area is where your stats for your character are raised and "built". Building your character is a common term for raising your stats. When you are in shop mode, the contents of that Shopkeeper will be shown instead. This goes for looting bodies as well.
This is your personal inventory. It contains all the items that are currently on your body. The other personal inventory you have is the bank. The bank being the most safest as you cannot ever lose anything through it.