WAV files are uncompressed and average about 10Mb per minute, whereas MP3 is compressed at about 1 Mb per minute. You need a WAV to MP3 converter (have a look here: click here )
What soundcard do you have? My Creative Audigy 2 has a "Wave Studio" program included which allows me to convert recorded WAV files to MP3 format.
Cakewalk Guitar Tracks 2.0 is well recommended (haven't tried it myself) click here and I believe will convert to MP3.