On the bottom center of the screen is your action bar, made up of a series of empty slots. Pressing the key associated with that slot activates the power or item placed in that slot.
To add a power to your action bar, drag that power from your power panel into the desired slot.
To add an item to your action bar, drag that item from your inventory into the desired slot.
The slots on the far left and far right of the action bar are for your left and right mouse buttons, respectively. These slots start with powers assigned to them, but you can assign new powers to these slots if desired.