I read it on the back of my old halflife box that you could download and use wolrdcraft to make maps. My very first map was a hollowed cube with a xen tree and a light in it.
One of my earlier maps entailed a house with a pool and diving board. There'd be secrets via func_illusionary doors in plants that you could go inside and other random stuff exploring entities. I remember making a map where you started out in some weird part and then fell way down into a night skyed area with a little multi story building. It had an elevator and each level had different stuff in it. One level had giant gummy bears that were func_pushables. Another had, well I don't remember
I remember another early map that was "shooting practice" where you start out in a room full of guns and walk down a hallway full of doors. Each door lead to a small room/ environment for a different monster from halflife. You would push a button and for example a headcrab would appear and you could shoot him. His environment was xen like with waterfalls and weird lighting.
Another interesting map was for the map a machine contest. Though I never finished and submitted it, it was an old abandoned merry go round/ carnival area. You turned it on from an old control station that had a tv in it with an animated texture of a badly drawn porno playing... When it turned on one of the horses ran off of the ride and crashed through the ground. You then jumped down the hole into a sewer that took you around and then under the merry go round where you could see the rotating shaft and some extra horses and neat stuff.
I've made probably three maps where you eat shrooms and begin to hallucinate. Things happened like your ceiling growing and shelves beginning to float allowing you to move upwards to a revealed door. Strange lighting occuring. A picture of the devil appearing and his nose growing. Yea creepy stuff...
I've even mapped a giant skull that opened and closed his jaw. You would stand on a platform and jump into his mouth and fall down his troat. I also made a map with water and a ship rocking in the waves. You jumped onto a floating box and into the ship and you could go in a door on the ship but it teleported you to a big interior area with bookshelves and a crossbow "harpoon".
Yes, I made tons of maps. Hammer is really awesome. Didn't release a map until torture. Was pretty young at the time and got some unfair criticism that I didn't know how to handle ie. ignore.
Forgot to mention, I too randomly found twhl, probably from google. Learned almost everything I know about mapping from the beginning from this site, I really do owe a lot to you guys.
My advance is that no matter how good you get at mapping, halflife has some unfair weaknesses and limitations. But, it compiles extremely fast. Get good at hammer for fun but if you plan on going into architecture or game design, look into more advanced programs that can make complicated geometry. I am using rhino and the things I can make in it make hammer look like ms.paint. But the experience i've gained from the wntire experience is quite un attainale elsewhere. So, keep at it. Learn it, master it, and keep doing it. Make maps for their playability if they are to be played along with their aesthetics. Look at surf_icebob for counterstrike for example. It has tons and tons of things you can do "surf wise" and it looks so simple. This took lots of thought, not just random placement of stuff.
Oh, inspiration for me comes from other video games and life experiences. I would point mostly at super metroid, banjo kazooie, zelda"s", megaman2 and halflife. Note their musical qualities as well. Very inspirational.
Edit: Forgot to mention. For the longest time I didn't know hammer could mirror objects vertically and horizontally with ctrl i/l. If you are ever making something symmetrical, construct one halve and then mirror it. I made my entire hunter ship from metroid prime without knowing this...
Also, hammer likes triangles.
And utilize the hell out of vertex manipulation. For everything!