There are many examples and tutorials for App Designer available in the MATLAB documentation.
Here is a quick video tutorial explaining the main features of App Designer: https://www.mathworks.com/videos/app-designer-117921.html
Here is a tutorial on how to create an example application:
Here are several examples of how to implement specific App Designer features:
Note: App Designer is only available starting in version R2016a. For earlier releases of MATLAB, you can find tutorials and examples on the older app creation tool, GUIDE, in the MATLAB documentation.