As hsawires mentionned, what you're looking for is the Crop tool.
It looks like this in the toolbar.
You can also input hard or relative values through the "Image -> Canvas Size" menu.
Checking the "Relative" box will set the values to zero and any value put inside the boxes will ADD to the current canvas size. You can also use a "-" before the number to substract from the canvas size.
The box with the arrows helps you choose in which direction you want the canvas to expand. By default, it is centered and will extend equally on all sides. I'll briefly explain, but it's much easier to understand by playing around with the tool in a blank canvas with a colored background and a few shapes laying around.
Clicking the "top center" arrow in the menu will create all new pixels at the bottom of your canvas.
When you look at the state of the arrows, it's easy to understand. This screenshot was taken after clicking on the top center arrow.
The empty space under the bottom arrows is the new pixels that will be created.