To know the password of the wifi on the computer is already connected to the network, you should click the right mouse button on network connection and select "network and sharing Center". Open a large window where you find the link "Manage wireless networks" (left menu). Select the desired Internet connection, click the right button and select "Properties". Go to the tab called "Security" and tick the "Show characters". Then, under "security Key" will appear password of wifi.
If your computer does not have the ability to manage wireless networks, then use the following method. Opening the window of available connections, click the right mouse button on the desired object and then click Properties. Then proceed by analogy: the "Security" tab I want to show the characters and find out the password from wifi.
If your computer is not connected to Wi-Fi, you will have to use a power cord. Usually, it is bundled with the router (if not, borrow from friends or buy). Open any browser (Mozilla, Explorer, Opera, Chrome) and in the address bar, type the numbers 192.168.1.1. In the opened window type login and password "admin". Then go to Wireless, select Wireless Security and look for the line PSK Password, where will be displayed your password.
Hacking passwords from other people's wifi networks is illegal.