I remember a quote by some smart guy who said more or less, "Creativity won't always come to you, you have to chase it with a bat". Comparing my first map to the content I make now, they all came from an idea I got from movies, images, my city, music, video games, or books. I respect everyone (from an artist pov) who releases a map because not everyone has the discipline to even get that far. Best advice I can give is get inspired by your surroundings and build upon those ideas. Fill your map with sections tied to your locale, the layout often comes from that. Look up tutorials, TF2 must be the best place to start Source mapping because of all the resources and dedicated sites like tf2maps to help you.