Click Themes to the right of your screen.
Experiment with different themes, colours and fonts to find the one that is the right fit for you.
If you hover over your header another menu will pop up.
Click Header type and experiment with the different types of headers. The default is Banner but you may prefer one of the other styles.
Hover over your header.
Click on Change image.
Find the image that you want to use - this could be one that you upload from your device, or select by searching within your Google Drive or by using Google Search.
Click on the image you want to use.
Click Select.