Well with resourses in these sites I think you can get started...
If you don''t know how to use worldcraft,try this,it also carries some hl mapping tutors along!
There''s a hl mapping message board in Sierra community(HL part)
planethalflife also has one.
groups.google.com also has one.
7.Some other sites:
8.Play great maps!
download USS Darkstar and They Hunger from his site.
9.Research others'' levels,work hard and you''ll succeed,there''s no secret way here,I think.Read articles as many as you can,test all the mapping gems in your worldcraft,Mmmm,what an interesting work!I really envy those who can spend time on mapping.