Can't get any sound when I plug Midland MX-5

Can't get any sound when I plug Midland MX-5 Speakers into my "new" (refurbished) IBM Pentium 4 Desktop PC...
I used to have a different PC, and this speaker system was BUMPING with that one. It was my old Dell PowerEdge 1300... I never had one problem. But that computer died on me, and a friend bought this one for me--strangest thing is, though, the only sound I can get out of it is some sort of internal speaker (?) which is not loud at ALL... and when I plug ANY speaker into the (what sure looks like a) sound card, there is NO SOUND! Headphones will work for me, and I can get a mic to work separately, but no external speakers, or what am I doing wrong? Maybe I need a refresher on how exactly everything is supposed to be plugged in? What goes where, exactly, in the speaker itself and the computer? Do I need some sort of new drivers I need to download somewhere? Is it a soundcard issue?

I am not the most computer hardware/software savvy person around, so step by step help would be greatly appreciated. I am about to tear my hair out. Aaarrrgghhh!! (I miss my music!!!)