Find out in a brief overview which describes the project. More detail is provided in the Introduction to Geant4, and in full journal articles NIM A 506 (2003) 250-303, IEEE Nuc. Sci. 53 No. 1 (2006) 270-278 and NIM A 835 (2016) 186-225.
Building an Application
Once you have installed the toolkit, the Application Developers Guide will provide information and examples for building your own simulation application.
As problems arise, try browsing the user forum to see whether or not your problem has already been encountered. If it hasn't, you can post it and Geant4 developers will do their best to find a solution. This is also a good place to discuss Geant4 topics in general.