You can put your spells or frequently used items on the action bars at the bottom and sides of the interface. You can activate them by either left-clicking on them or pressing the corresponding hotkey. A hotkey is a button on your keyboard that will automatically execute the command it is tied to. Hotkeys can be remapped in the Hotkeys section of the Settings menu which can be accessed by pressing "ESC".
All spells will immediately be put onto your action bar when you first use them, and you can move them around by simply dragging them from one slot to another. You can also add spells or items to the action bars by dragging them from your Spellbook, bag, bank, or Character Info window to the desired location on an action bar. To remove a spell, simply drag the selected spell off the action bar.