Facepunch S&box wiki[wiki.facepunch.com] will teach you how to make a gamemode for s&box and how to set everything up. It has not dozen of information, yet still important. First of all search for your answers there.

Since HL:A and S&Box share the same engine (even tho they use different programming languages), Valve's developer community will be a good guide for making maps, S2FM etc.

BUT NOT SCRIPTING, S&BOX DOESN'T USE LUA!

Dota 2 was litteraly the first game on Source 2 and it's been 6 years of people creating something on this engine. Even tho it's mostly highly specialized for Dota 2, you still can find something useful Here.

If you finaly got access to S&Box, then probably you are already joined Official S&Box Server[discord.gg], if you didn't, I think you should.

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Also there is a big Source 2 Modding Community[discord.gg] which includes both Half-Life: Alyx and S&Box. It's the biggest one about Source 2 and you can always ask your questions in #help.