Create a Goldsrc-Mod
While making mods for 
GoldSrc is arguably less-complicated as its successor, 
Source, there is still a bit of work to be put in.
Creating a GoldSrc Mod
Creating the folder and the liblist.gam file
All GoldSrc mods begin with making the liblist.gam file, a mod that lays out certain information about the mod like the mod name and what map to load on New Game. For more information, see Layout of Liblist.gam. Although first, it needs a folder. In your Steam folder (ex. C:/Steam/steamapps/common/Half-Life), create a new folder and name it whatever you like. Make sure it is alongside the game's executable (ex. HL.exe) and its folders (ex. valve and valve_hd).
Next, go into said folder and create the file liblist, with an extension of ".gam". Alternatively, you can copy the file from any GoldSrc title or mod and edit it. Under the value "Game", put the title of your mod and under the value "type", put the type of mod it is (ex. singleplayer_only).
Lastly, your mod needs some client-based files from the base game, which are all stored in the folder "cl_dlls" and "dlls". They can be found in any Valve or mod content folder, as mentioned previously, like, once again, "valve". Copy both folders and paste them into your mod folder. Restart Steam and when it reopens, in your mod list, you should see your mod in your game list. Congratulations!
Now, for what to actually do now that you have a mod, see below.