If you're talking about writing custom game code, then the wiki here contains a guide that looks pretty comprehensive:
Half-Life Programming: Getting Started. What exactly do you want to achieve?
always thought "that's way too much for me" (I was mostly used to CS mapping).
I also started out as a mapper, years ago. I eventually started experimenting with the game code and I somehow managed to make the MP5 launch a grenade with each shot, but I couldn't really do much else. Now, years later, I do programming for a living... So yeah, give it a go, and don't get discouraged if things seem complicated at first. Feel free to ask for help whenever you're stuck.