The Inventory can be accessed by pressed the " I " button. Its made up of a 10x8 screen giving you the ability to hold 80 items at once. When your inventory is full you will need to find a place to store things before you can pick up anything new.
The hot bar will add anything automatically the 1st time you collect it if there is a free slot available. It consists of 7 tabs labeled F1 to F7 (Indicating the shortcut key to jump between the various tabs at speed)
You can navigate the hotbar using both the mouse wheel and the Page Up and Page Down buttons to cycle through your items.
You can also use you numbers on the top of the keyboard to quick select what item you would like to use. Right Clicking on any hotbar icon will remove it.