Start by going in the Control Panel (Start+Settings->Control Panel) and find the network icon as shown. Double-click on it: the network configuration panel comes up.
In the displayed list, you should find a protocol named TCP/IP. If not, you will have to refer to your system documentation to find out how to install it. If it is already there, select it and click on “Properties”.
This window will enable you to set up your TCP/IP parameters. Your system administrator will tell you if you have a static IP address or if you are using DHCP (automatic IP address). Click on the Gateway tab.
The rest is child's play! Fill in the blanks with your gateway's IP address (i.e. 192.168.0.1, in our example). Click the Add then the OK buttons.
You will need to reboot your computer, of course. Once this is done, find out if you can reach the rest of the world.