So you want to get into coding.
I started it in 2015 by getting Visual Studio 6.0 (it's from 1998, so I thought it would be good for Half-Life, LOL) and downloading Half-Life SDK 2.3 from ModDB.
That is wrong. VS6.0 is ancient and it's the best to use a modern development tool.
Since I don't know what's the best place to obtain the HLSDK (I'm using the edited SDK2.3 that's meant for VS2008, and I'm still using Visual C++ 2010 Express), we'll have to wait until somebody answers that.
If you want to get into [insert game here] coding, you must learn the basics of programming to at least understand some things. Trust me. When I started, I had no idea what is what. It's sort of like seeing a trigonometry test as a 5-year-old kid.
But also, if you implement new entities in the code (e.g. func_something), yes, you'll have to modify your FGD file. I use Notepad++ but regular Notepad can do it just fine.