When you have to create a new Geant4 application, you have to preapre your CMakeLists.txt file and your main file.
If you do not have particular requirements, such as the need of using root or other external libraries directly in Geant4, I suggest you to open the CMakeLists.txt file of a standard example and replace throughout the file the name of the original app with a name you prefer. This name has to be also the name of the main file of your custom application.