I would recommend reading through this guide (
https://forums.civfanatics.com/threads/modders-guide-to-civilization-v.385009/) for Civ V if you are brand new to modding. It's not completely up to date but it will give you a good introductory overview...and a good deal of it is applicable to Civ VI as well.
If you're looking for tutorials in general xml or sql, a quick Google search should bring up multiple tutorials and references. Honestly, a tutorial reviewing the languages themselves is a bit outside the scope of this forum, especially given how many resources there are just a search away.
If you want to learn more about xml and sql specifically as they relate to Civ VI, my best suggestion is to start trying things. Poke around in the game files, look at the different tables and what kind of data they hold. Try tweaking some values in the files and see what happens in the game. If you break something you can always verify the integrity of the game cache and Steam will redownload any files you messed up. Also, download other mods and take them apart...this is probably the single most helpful thing you can do. Many mods are very simple, so it's easy to figure out what's going on.
But at the end of the day you won't receive much help from the community if they don't see you trying. So, try some stuff, see what happens, and if you have
specific questions come back and you'll receive help.