You simply need a way of getting the sound into your PC, and a way of processing it.
Depending upon the outputs from your decks (or mixer) and the 'line in' socket on your PC you'll need a suitable lead to make the connection. (Your local Maplin store can supply whatever you need).
Then you need the free software which almost everyone uses. (Even if you buy something which is advertised as a device specifically designed for converting vinyl tracks to digital, it will almost certainly come with this program):
http://audacity.sourceforge.net/
You should note that, for licensing reasons, Audacity can't (in its native form) export files into MP3 format. However you simply need to follow the instructions, here, to get round that problem:
http://audacity.sourc...?s=install&i=lame-mp3
Also, while Audacity is a fairly intuitive program, I strongly recommend downloading and reading the documentation for it:
http://audacity.sourc...et/help/documentation
and reading this tutorial:
http://wiki.audacityt...rds_to_computer_or_CD
Chris