The combined Modem/Router replaces the USB Modem that you have been using. You need to go into the router's configuration pages and set it up to connect to Tiscali, or you may have a CD which includes a wizard to guide you through the configuration.
Once set up, the main computer can just be plugged in by ethernet cable and it should then have internet access.
With the Laptop, run its wireless software to scan for "Available Wireless Networks", highlight the one that matches your router's SSID (network name) and select the option to "Connect" to it. Once "Connected", the Laptop should also have internet access.