Download the latest Geant4 User’s Guide, open Chapter 2, and take your first step.
And when you finally get that G4Trajectory to draw a beautiful line through your detector, you will thank the manual—even if you just used it to find that one parameter name you forgot. geant4 manual
The documentation is structured into several key volumes to address specific simulation needs: Introduction to Geant4 Download the latest Geant4 User’s Guide, open Chapter