LÖVE

I don't know what I'm supposed to open in order to code.

Jeeper has some tutorials on youtube, and there's the tutorial written by sheepolution. Sheepolution's one has installation instructions for love and ZeroBrane as the code editor of choice, while I think Jeeper uses Visual Studio Code. Choice of code editor ultimately is up to preference, though.

main.lua isn't working. It's just main.lua.txt

You have to make sure that you're saving your file as "main.lua", not "main.lua.txt" (without quotes, naturally), or manually change the file extension after the fact (which you shouldn't have to with any decent editor). Some text editors may try to add the .txt file extension unless you specify otherwise. To help aid with this issue, it may also be beneficial to make sure your OS doesn't hide the file extensions (the ".txt" part in this case).