Hi! I just recently got into modding Civ V over the last couple weeks. Changing things like yields from buildings, tiles and social policies, is very easy (relatively speaking) using just XML alone. I'm just getting into Lua scripting so I can actually implement new logic into the game and add interest religious beliefs and social policies and such,
which very much seems to be the hard part.
You need the Sid Meyer's Civilization 5 SDK.
You can download it on steam, by going to your games and checking the box for tools. You will also need some software from 2010, the Visual Studio 2010 Isolated Shell, and unfortunately Microsoft in their infinite wisdom has decided to incorporate a subscription model for their software development tools. I was able to download it for free since I am a student, so there are workarounds.
Once you can actually start modding, I recommend you start by trying to change some building yields. Very fortunately for us, you can actually just open up XML files from the actual game in Notepad++
(you should get that as well if you haven't) and see how things are structured and use that as a guide. You can find them on your computer under something like
C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization V\Assets\DLC\Expansion2\Gameplay\XML
There are very old YouTube tutorials that can help you, try starting with this
From there it is a lot of googling forum posts, there are some very good guides on here.