Assuming you mean the network encryption key needed to use the network rather than the password needed to log into the router's administration pages, if it isn't on a label on your modem or router, the likeliest place to find it is in those admin pages. Exactly where in those pages varies from brand to brand. If you don't have a router, the following should apply to your modem itself.
Basically, to get into the admin pages, do this:
1. On a computer connected to a wired port on your router (wireless access to the admin pages should be turned off as a security risk), enter the router's local IP address in your browser's address window.
2. Poke around, especially under wireless security, until you find the encryption key. Copy it down EXACTLY. Copy-and-pasting it into a text file (Wordpad is generally best because it's on almost all Windows machines) is even better, as it can be copy-and-pasted from there into whatever needs it. When you see it, you'll understand why this is a good idea.
To find your router’s IP address in Windows:
4. Click on the Start->Run button.
5. In the text box, type
and press Enter.
6. A black "DOS box" will pop up. In it, enter
A bunch of gibberish will fill the box. Next to "Default Gateway" an IP address will appear. This is your router's IP address.
Hope that helps.