How can I add a sound card to a computer that has on board sound card?

Go into the device manager and disable your current sound card, and then install the new one. Your computer should automatically see the new one and you should be able to use it right away. If you have problems with not hearing anything after installing the new one, go into the control panel to the sounds and devices settings/audio tab and be sure that your default device (new device) is the one listed.