if u have a microphone socet in the back of your pc, it will be shaped like the headphone one (the small ones u get on pcs and mp3 etc)

and u have an amp with a headphone socet (it will be big and lead shaped). u basically have all you need. yopu will need to buy an extra lead. and an adaptor that converts from a lead size to the small mircrophone size. then plug the lead in to the headphone on the amp. stick the adaptor on the end, then plug thi8s into the pc microphone soccet.

you should have one of these microphone soccets although it might be round the bak of the pc.