It's the same password that you used to connect the computer to the wi-fi network. If someone did it for you and you have no idea what the password is, you can find it out as follows:
1. Open
Network Connections: on
Start menu or
Control Panel, type
view network connections and click on the result that matches.
2. In the window that opens, double-click the wireless connection of your computer. This will open the
Status of that connection.
3. Click the
Wireless Properties button and it will open yet another window.
4. Click the
Security tab and there you'll find the password for that wi-fi connection. The characters are hidden by default, so click the
Show characters checkbox to reveal them. Use that password to connect your phone to the wi-fi connection.