Or, you need a simple spliter to go with that stereo system. Just need to get one that goes from a headphone jack to typical AV cables. You can make one yourself out of an old pair of headphones and a set of AV cables also if you want. Then you just plug it in, done.
You should probably look into buying a better, probably used, speaker set. This one will not actually do true 5.1 surround sound. It takes the left and right audio channels, and then has the two speakers on the left play one channel, speakers on the right play another channel, speaker in the middle plays both, and the base just does base like normal.
If you want real 5.1 surround sound, I got my setup for $5 at a yard sale and is a much higher quality one actually (few years older, but speakers are basically magnets, not a lot of innovation). You will need one that either users SPDIF, HDMI, or multiple of these spliters for 5.1 surround sound, as opposed to the current system you want to get that only has two channel inputs.