Google is your friend, my friend :P
EDIT: if you're using windows 7, to get to a map, the path is..
C: --> Users --> your name (or whatever you use for your username) --> AppData --> Roaming
Note that the name of the folder is .minecraft, so you can take the lazy way out and just do a search for it on your computer.
Your single-player maps are in the Saves folder.
To upload it to the internet... well you gotta go to a site that allows you to upload stuff.