On a PC running Windows, look for a narrow, metal plate with several small, open connectors (holes) in it. This is the sound card. One of the holes will most likely be labeled Audio, Audio Out, Speaker, Speaker In, or a similar term that indicates the connection provides sound to the external speakers. On older PCs, the hole may simply be labeled Line Out. On current models, the connection is often labeled with a small icon that looks like sound waves emanating from a tiny speaker.
When you've found the proper connection, plug the metal pin on the end of the speaker cord into it, making sure it goes in all the way. Press firmly, but don't force anything where it doesn't want to go.
Double-check the volume knob to make sure it's turned all the way down. Now plug in the speaker power cord if applicable, and press the power button to turn on the speakers. Once that's done, turn up the volume slightly.
Next, turn your computer on, and you should hear a tone through the speakers as it starts up. If you don't, there's a good chance you need to adjust the sound volume on the computer itself. Check out Adjust The Sound on Your Computer to find out how.
Hope this helps,