Help!!Please with connecting speakers to pc??!!?

I barely got my computer fixed.So I have to connect everything back.Its a desktop computer.And now I can't put the speaker right.I tried and tried but all it came out was like a scratchy sound like as if a radio was on but no music nothing.There's a green cable and a black one!!How can I restore it??It worked perfectly fine before taking it to fix it.Please help?~