To begin working on anything, you need one of two things: a new image or an existing one that you opened.
To open an image, select File > Open and open the image you want to work with.
To create a new image, select File > New

For the purpose of this demonstration we are going to be creating a new image. You should create a new image at this time so you may follow along.

A dialog should pop up prompting you to define the elements for your new image

• You may name your image now, or later when you save the image
• You can fill in your own pixel dimensions for height and width or select a size from the pull down menu
• You can change the background for your first layer. Selecting white will create a blank white background. Transparent will have a checkered background that is not visible once you export your image

You are now ready to begin working on your new document.