Tbh as some have said, start with Lua, then continue when you feel ready.
Alot of people have proven that you rarely have to do source edits (atleast with 1.x seeing to the functions we have available).
You have to know atleast basic C++ if you wanna do changes to the source code, beacuse we can't change everything via Lua, even if we are moving more and more to the Lua API.
But if you wanna be able to manage a server by yourself (good luck haha) you have to know ALOT more ...
* The structure of your database (not a must but it helps if you know the basic stuff here)
* HTML, PHP, CSS, SQL and maybe even some JavaScripts to be able to code a nice website, atleast a layout.
* As I said, more than the basics of Lua to be able to fix bugs, optimize your scripts (this is a must if you get alot of players), learn the variables you have aswell as what you can do with them.
At a later stage you need to learn how to use C++, same thing here, wait till you actually need it.
But if you wanna compete with the most updates OTs you need to find a way to know what Tibia changed in the protocol for the next version, when you know that and can code the needed C++ code for it you can update to the latest client version.
This is usully pretty easy but can be a real pain at the same time.
I would start with Lua, since most of the time you will spend coding is ... Lua.
Then the website comes, here is where you have atleast have to learn HTML, PHP, CSS.
"Finally" you can start learning C++.