can you suggest some sites, authors or books or any other medium...
to learn music theory..
well i am an starter..ameteur...
so
some advise for a begginer??

http://www.murraystate.edu/qacd/cfac/music/MUS109e/intro/index.htm

there are a bunch of free online sites. you just have to google in music theory and hunt around for one that you like.

The very best way to learn and to forever improve is to get with and jam with other musicians.

if you havent heard of this you should check it out, you may find something you really like http://www.musictheory.net/

http://www.emusictheory.com has some good theory drills for guitar and other instruments