There are two ways. With Lua scripts, or editing the source code in C++
Lua is way better for the users and can be put on the script manager, but it cannot do everything.
C++ forces users to use an entierly diffrent executable / version of the game. A lot more things are possible but it also requires a lot more maintenance, you have to update your mod when the vanilla game updates.

Anyways I recommend using Lua
You can learn it from the official documentation here: https://www.lua.org/pil/contents.html
For actual modding in tpt see the Lua API on TPT wiki page here: https://powdertoy.co.uk/Wiki/W/Powder_Toy_Lua_API.html