There is no simple answer to your query.
You wish to modify the tank shell explosions.
For a self made tank you would do this in a config.cpp for the tank.
For the standard in-game tanks you would need to modify the config.bin......which is the config for the whole game....everything more or less.
To modify the configs you would need to know about how configs are constructed.
You can do this either, by reading a tutorial, or examining other configs (BIS's commented configs springs to mind).
The game config is.....config.bin, and you need a program to convert it to a config.cpp in order to edit it. This is not for the faint hearted, if you ever do this make sure you keep a backup somewhere safe.
In short nobody can teach you how to write or edit configs via the forums, they can only really answer specific queries about it, not full blown classes
Planck