Go to places like
interlopers.net
hl2world.com
And others like it, fire up hammer and start learning. It's a pretty well built editor and doesn't take too long to get to grips with depending on your knowledge of how games work (trigger boxes etc).
Making good maps does take a long time and it's best to plan everything out on paper, also learn about optimization before ploughing in to building as this will save you later.
When I used to build maps I would build certain sections in their own map so I could get everything perfect and know that any problems were with the exact bit I was building rather than some other entity etc. Then I would move it in to the main map when I was happy with it.
Good luck.
