Thimble is divided by the right pane, which is a live preview of web page that you're creating, and the left pane, which is the HTML and CSS code that is putting the page together.
Hit the Remix button
on a starter make and follow the instructions written in the code.
Pick different areas of the code to modify and see what happens. When you hover your mouse over certain areas of the preview pane, you'll see that the section of code responsible for that area is highlighted for you on the left side.
Don't forget, you can remix anything that is created on webmaker.org, so if you see something created by others that you also want to make, hit the Remix button and see how they did it!