If you are trying to connect to the internet via your wireless network, then you should first attempt to connect to your router and see if you can locate the security code. To connect to the router, open a browser and enter either http://192.168.0.1
(your router's documentation should provide you with the correct IP address). To connect to this page, you will also need a user id and password. If you did not change the defaults, these should also be in your router's documentation. If you did change the defaults, you're probably looking at a complete reset (back to default settings) and rebuild of your wireless router/network (should also be covered in your documentation).
If your system is wired directly to a router, then you need to download and install the Windows 7 LAN drivers for your network card/motherboard (as 4745454b states).