Music for guitar
Check out http://www.musictheory.net
for some excellent presentations explaining the theory of music with exercises that will help you to learn to read music.
Download and install the TEFV program to view music for guitar written in .tef format.
and go to http://freetabs.org/ and then to http://freetabs.org/listtabs.htm to download the .tef files
Dirk's Guitar Page and Free-Scores have a huge library of music in .tef and .pdf formats which you can download
You can learn to write music using the free - open source program ' MuseScore' which you can download here - and you can watch music played from the scores uploaded through the search bar at the top of their page here.
To read music scores saved in the .tef format , download the tefv.exe program here . Search the internet for guitar music scores will get you to sites like : http://freetabs.org/