How do you load a *.bsp into hammer?

You don't. The Hammer editor can only open certain types of files, and then you use compiling tools to compile that Hammer map into a game-useable .bsp file. I suggest that you find a good information resource on HL1 mapping, because they're easier to find than HL2 mapping resources and they're fairly similar. The VERC collective is probably a good place to start.

http://collective.valve-erc.com/

As a side note, there are decompiling tools for HL1 .bsp files. However, they are not very good, and they are EXTREMELY frowned upon. If somebody finds that you have been decompiling maps to steal somebody else's work, you will quickly find your life very short and pain-filled. :)
 
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