Tophatwaffle, 3kliksphilip, and world of level design are all great resources.
Hammer ++ is absolutely amazing, but hasn't been released for csgo yet.
You will have to use normal hammer for csgo, you can download the sdk by following this https://developer.valvesoftware.com/wiki/Counter-Strike:_Global_Offensive_Authoring_Tools#Downloading_and_Installation
If you are considering really getting into hammer I highly recommend keeping your custom assets in individual folders to prevent auto packing issues in the future. If you don't, you'll end up with map files packed full of unnecessary textures which will make it too large for fast dl. This is the one thing I regret not doing when I started. You can find a tutorial here https://www.tophattwaffle.com/custom-content-without-contamination/
Mapping discord are a great resource. You can always search old conversations and ask hammer pros when you get stuck. Here's my list of discords;
https://discord.com/invite/unqF28r this is map core, they're more focused around clean competitive csgo maps, but are great for learning optimization and detailing along with modeling
https://discord.gg/sourceengine the source engine discord, the resources channel is an ongoing list of helpful resources
https://discord.gg/zVRDgWU the hammer editor discord, this is the best place imo to ask about custom community maps
A great way to learn is by finding something you want to do on an existing map and decompling it to see how they made it inside hammer. For that you'll need bsp source https://developer.valvesoftware.com/wiki/BSPSource
The default compiler is alright, but I recommend compile pal since it's easier to use and let's you adjust how many cpu cores it uses. https://compilepal.ruar.ai/
You will also want a packing program. Compile pal has an automatic one, but sometimes you'll have to add or remove an asset manually. For that I recommend VIDE https://www.tophattwaffle.com/downloads/vide/
If you're looking for free custom textures this is a good place to start https://www.textures.com/category/brick/1
You'll have to convert them to vmt, I wrote a guide here
If you have any questions, feel free to message me on discord