I need sound help!

Hello,
im having trouble with my sound,
sometimes it works sometimes it doesnt!
i need a step by step reply. im not great
with computers. but ushaully the problem
comes when i plug in my headphones
and then when i take them out it wont
work for either. My computers not on mute,
or anything this has happened several times to me
before. Everyone says sound card, i have no
idea what that is! and i dont have the cds for
this comptuer or anything. but PLEASE try
your best tohelp me, i would really appreciate it.
My computers a WINDOWS VISTA tm BASIC.