You should do tutorials and practice a lot.
Here are some sites, but feel free to search for them on your own:
Focus on two thing if you are beginner.
1. Basics Concepts of OOP.
2. Learn Data Structure
3. get helps from these link it is not help you compeletly but its help you.a
https://www.geeksforgeeks.org/
https://www.thedshandbook.com/#:~:text=A%20data%20structure%20is%20a,use%20depending%20on%20the%20data.
https://erlerobotics.gitbooks.io/erle-robotics-cpp-gitbook/content/object-oriented_programming__oop_and_inheritance/exercises_oop.html