Assuming you have Windows XP click Start>Network Connections. In the Network Connections window look for your wireless adapter. If it says "disabled" right click on it and choose "enable". If enabled your laptop should automatically search for available networks.
You may have to enable wireless access in your Belkin router if you haven't used the wireless feature before. If your other computer is already using wireless then you should be able to connect. If not then possibly the router's security settings may be blocking your new laptop. To access the GUI for the router from the connected PC enter http://192.168.2.1
into your browser's address bar to access wireless and security settings.
If using wireless you should at minimum use WEP which uses a password that you set up on the router and any PC's that you want to connect to it.
For more info RTFM! (i.e. see you router's manual for more detailed info.)