Can not run Apache (sudo apachectl start) on OS X 10.8

I want to start Apache on OS X Mountain Lion 10.8. I type this on the terminal:

sudo apachectl start

and press enter. It asked me to enter password. I don't have set any password for my system. So I press enter again. But it does not accept my blank password. After 3 times, it quits the execution of the sudo command.

I type: "httpd -v" at the terminal and get the following output:

Server version: Apache/2.2.22 (Unix)

Server built: Jun 20 2012 13:57:09

Can you please tell me, what should I do to run Apache, using sudo command? I also set a password for the system in the Preferences > Users and Groups, and I am sure that I have administrator privilages (the "Allow user to administer this computer" is checked).