Let's start with the condition and this brings us to a very useful feature of logic apps; while you can add a step to the bottom of the flow, you can also insert steps and conditions inside an existing flow. The area between each step hides a plus symbol in a circle; if you hover over that area, it will appear and can be selected:
In this case, we'll select the area just above the posting of the tweet. Once you select it, a context menu will appear, allowing you to Add an action. If you select that, you'll be presented with the (by now) familiar dialog, allowing you to select an action. In this ...
