Welcome to the neighborhood!
You're new so we'll let you live...
this time.
To open a BSP in an editor you will have to decompile it, which is uncool. Search the forums for "decompile" and you'll see the discussion on why some people
(like me) think decompiling is bad. If you actually want detailed info or tools I'd suggest hitting google.com.
I'd say skip the decompile and read the tutorial section, many have example maps. There are even more examples in the vault. Mounds of information, just waiting to be absorbed. Go... absorb... map!