This is part 16 of the Photoshop quick tip series where you will learn how to add more canvas space around an image.
You need to firstly have an image open in Photoshop. Then from the menu bar select Image > Canvas Size and you should see the following dialog window should open up.
To increase the canvas size, you simply adjust the width and height settings to whatever size you like. You can also set the anchor point of the canvas which will specify the direction where you want to increase the canvas size. So, for example if you want to increase the canvas in the left direction then you would select the right anchor point.
For more quick tips, checkout the post on the Photoshop quick tip series.