Stereo Speakers on PC

I have a Sony Stereo. It’s very nice (like 80 cd changer or something) but the problem is, I don’t use CDs anymore. Is there any way that I can connect them to my PC’s speaker output? The speakers connect with wires. Each stereo has two wires, one black and one red.

Also, can anyone recommend a simple and cheap device that i can use to connect my guitar to my mac?

Thanks for your help.