Tools & Plugins:

Hl-Texture tools - Wad & Sprite Creation Utility
Wadmaker & Spritemaker by captiain P (new cmdline wad & sprite creation utility
GoldSrc multi-tool extractor
Half-Life SDK (can also be downloaded through steam)
BSPGUY by w00tguy - A BSP editing/ripping, viewing BSP multi-tool
Nem's Crafty BSP Viewer Tool
ModDb's Half-Life Section
GameBanana's GoldSrc General Tools Section
GameBanana's Half-Life Tools Section
GameBanana's CS1.6 Tools Section
Sourcemodding.com GoldSrc Tools Archive
RIPENT - BSP entity ripper
BSPedit
Half-Life Zone Editing tools (archive link)

If you are coming from a source engine background this quick reference will show common equivalents:

func_brush --->  func_wall
func_detail ---> func_detail (loaded with zhlt.fgd)
I/O system ---> multi-manager (turn off smartedit to add outputs)
prop_static ---> env_sprite with model path or cycler_sprite or item_generic (OPfor, CZ & sven entity)
prop_dynamic ---> monster_generic (with scripted_sequence to activate animations)
nodraw,tooltextures,ect -----> zhlt.wad  (get Vluzacn's Map Compile Tools  ).  also Nodraw = "NULL"

Models & Wads downloads:

Wally wad editor
17buddies GoldSrc Model Downloads
Gamer-Lab GoldSrc Content Downloads Resource
The Half-Life High Definition Pack

Coding:

Solokillers updated HLSDK that is compatible with VS2017
Original HL SDK by Valve (works on Steam, could fail on WON)
HL SDK 2.3, the one that works best with WON
Sourcemodding programming tutorial list
Engine notes from metamod.org
Wavelength coding tutorials

Some notes from various people

Learn the basics of C/C++, most of the problems people have are basic C/C++ mistakes and not realted to HL1
Start small, go bigger. Another problem of "beginners" is that they want to turn HL1 into Unreal Engine 4 without knowing how to make a simple entity and such.
Have patience, you can't program a new weapon in 30 seconds and expect it to work as you would like to.
Avoid copying/pasting code blindly without knowing what it does
Always make clean and documented code (don't be like Valve)
Use VCS if you know how to use them (git, SVN, Perforce...)
Making a MP mod? Support Linux for moar dedicated servers
ideally you should use CMake to manage the project files, it's easier to do cross-platform that way