The best way is to choose a template.
Go to Campaigns in the left navigation pane and filter the available campaigns.
Then hover over the one you like and click the 'Use This Template' button.
How do I change content and images?
Just click on the content and it switches to an editor.
To change images, click to the right of an image and hit the 'backspace' bar or switch to source code view and change the HTML code.
You can do this by clicking anywhere in the content area. A content editor should pop-up. Click on the "HTML" button on the top left corner, then edit the code.