In order to start the program, (assuming Windows XP) click on Start, Programs, Wireshark (Group) and click on Wireshark.
Once the application starts-
Click on Capture and go to Interfaces. Select the active network card. Now click on the Options button in the Window.
Make sure the Capture packets in promiscuous mode box is check in the Wireshark Capture Options check box.
Now click on Start (This starts the capture process)
You will now see packets of data that are now passing across the network. Allow this application to run for several minutes. Return to the menu and click Stop.
You can find packets by typing key words or the word password and hitting enter to find the packets that may contain the password.