There's no single "correct" way to set this up; what the permissions need to be depends on who should be allowed to do what to which files. Does anyone other than you need to be able to edit the web files? Is it ok if other users on the computer can read the web files (directly, as opposed to via the webserver)?
Here's a simple option that gives you full (read+write) access, lets the webserver read the files, and locks everyone else out of the web folder (note: replace "sandro" with your actual username):
sudo chown -R sandro:_www /Library/WebServer/Documents
chmod -R u=rwX,g=rX,o= /Library/WebServer/Documents
If you need other access rules for the web files, please clarify.