That depends on the modem.
If you have an HG659 modem and you have not changed its default configuration, the manual says that the WIFI network SSID is of the form "WiFi-XXXX" where XXXX is a 4 digit alphanumeric code and the key/password is printed on a sticker on the back of the modem.
Alternatively, you can log in to the modem and (assuming you have a HG659) go to Home Network/WLAN Encryption and click on the CheckBox next to "Show password".
If you don't have the HG659 modem, there is something similar that you can do.
By the way, the default ip address for the modem is 192.168.1.1 and you need to log in using the user name "admin". I can't remember what the default password is, but it may be "admin". Check the back of the modem and/or your modem manual.
You can also refer to this useful article.