Pretty easy!
Make sure there is a free set of inputs on the back of your stereo. This should be a label that says "Input" and two connectors (usually red and white). Alternateively, they could be labeled "Aux," "Tape" or "CD." This is the source that you will select on the front of your stereo when you wish to listen to sound from your computer.
Determine the length of cable you need. Measure the distance between the back of your computer to the back of your stereo. The cable generally comes in 3-, 6-, and 12-foot lengths.
Go to a home electronics store and purchase a Y adapter cable of the proper length. At one end the cable will have a plug to go into the headphone jack of your computer. (The plug will look like the plug on the end of any pair of headphones.) At the other end, the cable will have two RCA-type plugs (red and white). These go into your stereo.
Plug the computer end of the cable into the headphone jack on your computer. There may be a few jacks; the headphone one is usually green. Connect the other end of the cable to the free set of inputs on the back of your stereo. Be sure to match red to red and white to white.
Turn on your stereo and set the volume pretty low so you don't accidentally make a REALLY loud noise.
Select the input to which you connected the Y adapter. For example, if you connected to "Aux," press the "Aux" button on the front of your stereo.
Adjust the volume of your computer to around 80 percent. You should hear the sound of your computer playing through your stereo.