Creating a Screen is the first thing you will do with new App. To create a Screen, click the Add screen button located in the top of the App Outline column on the right side of the App Builder.
To rename the Screen, simply click back on the screen's name. Delete or add text as needed.
Choosing a Screen Type when creating a screen is important. The Screen Type you choose will determine how the fields will display to the user, what field types are allowed, the number of fields that will be available, and how the user will interact with the Screen. To select the Screen Type click on the screen and open the Screen Settings tab.
Most of the time a Labels Above type will be sufficient for you needs. However, there are other Screen Types that will make the device user's experience better. You can read more about Screen Settings here.
This video tutorial will walk you through the process: