sudo apt-get install ntop
#The above will install ntop, now set a password:
sudo ntop --set-admin-password

#Start ntop with the following command:
sudo ntop -u ntop -d -i eth0

#Be sure to replace eth0 with the adapter you would like to monitor. In my case it is wlan0.

#NOTE: Ntop is now running as a daemon in the background of your PC. Personally, I wouldn't make it a practice to leave this running -- it can take up a lot of RAM.

#That was simple enough. If all goes well, you will now be able to access the web interface:
http://localhost:3000