How to Reset Dock Items in Mac OS X
Your Dock is meant to be customized, so don’t worry about removing items.
If you do want to reset the Dock to its original configuration, launch Activity Monitor and navigate to ~/Library/Preferences/ in the Finder. Trash com.apple.dock.db and com.apple.dock.plist, then immediately quit the Dock from Activity Monitor.
You do this all at once with this command line (but note that it will delete the files instead of trashing them, so it is not reversible):
rm ~/Library/Preferences/com.apple.dock.{db,plist}; killall Dock
