The disk will have a floppy disk icon and may be untitled. The Toolbox Paintbrushes, air brushes and other specialty tools are available for you to create with. Click an icon in the toolbox to choose between the different tools. If a tool's icon has an arrow in the bottom right corner, click and hold on the icon to reveal other tools in that category. This will return the image to the state it was in when you last saved it.
Filters are another way to alter an image.
To use filters on your image, click and drag on the Filter menu to select a category and filter. Images containing layers can only be saved as a Photoshop file type unless you flatten the image. To flatten the image, choose Flatten Image from the Layer menu. Layers allow you to add to an image without altering what you have already done. It's like putting a piece of transparent canvas over a picture and painting on it.