There are a few links I recommend to people who want to learn Lua:

The first is this online book about programming in Lua: https://www.lua.org/pil/contents.html
I've never used it myself, but it seems like a very good reference that starts from the basics, all the way to the complex. I don't know if it's the best way to learn for someone who hasn't done programming before, but it's the best Lua-specific guide I know of.

If you want to look up a Lua function, I always use the reference manual: https://www.lua.org/manual/5.1/manual.html
This is just lists of functions, in case in the future you forget something.

Lastly, since you will be programming for TPT, you should check out what other people have made: https://starcatcher.us/scripts/
Find some simple scripts there and try to understand what they are doing and how they work. They will all use the TPT Lua api: https://powdertoy.co.uk/Wiki/W/Powder_Toy_Lua_API.html