Hi,
A good first step is to copy one of the basic examples and just play with it. Change an input file according to what the README says you can do, run it and see how the output changes. When you understand (roughly) how it works, modify the code to do something different that is of interest to you, recompile it and run it. For example, change the geometry and/or materials and see how the outputs change. Modify the readout mechanism, be it in stepping action or a sensitive detector to output a different quantity than the one(s) it presently provides.
When you get confident enough with that, you can start writing code for your own projects.
To use Geant4 successfully, you need to know C++. If you already do, that is great. If not, there are many introductory and advanced resources on line that can help you with that. For example,
https://www.w3schools.com/cpp/default.asp
https://cplusplus.com/ (currently down, but it was up a day ago)
https://www.cprogramming.com/ (covers C++ and C)
There are lots of others. If you are having a particular problem with some piece of C++ code, always remember that Google and Stack Overflow are your friends. For stuff related to Geant4, as you obviously already know, this forum is your friend.
To make sense of the examples, to be able to modify them, and to write your own fresh simulations, you will need to read the Geant4 documentation. The latest version (Geant4 11.0) is at
https://geant4.web.cern.ch/support/user_documentation
Most of what you’ll need at first is in Users Guide for Application Developers.
You don’t have to read it cover to cover at first, but you should skim it to find where the major topics are. Eventually, working on your own simulations will have led you to read most or all of it and the other reference manuals, as well.
All of this may seem a little daunting, but if you take it slow and methodically you will do OK.
Good luck.
P.S. There are training courses available, both introductory and advanced. Look for ‘Training courses and materials’ in the side bar of Geant4 Getting Started. I do not know who to talk to to find out what, when and where is coming up next.