how to customize open source pro

I am eyuel and system admin in bank industry,and I have pession for using programing languages to develop some projects,but when I tried to customize some open sources projects code it will be vast and not easy to get the exact point of the code to be customiez as I need. 

Any body who are volunter to show me the way plse sujest me how to become a good programe and silply customize the code in the open sources codes.

thank you for helping me and show me the way to be successfull like you.