The first step in learning how to read music on the guitar is to start with the notes in the first position. Learn the notes on strings 1 2 & 3 in the first position, then gradually add the notes on strings 4 5 & 6. It takes time and patience to master sight reading on the guitar so be prepared to spend many hours practicing. Don't try and learn them all at once. Practice easy to read material and work up to the more difficult songs.
Check out some of my Songs to Start Sight Reading It's good to try songs that you don't know, as well known pieces are too easy to memorise and play by ear. One of the biggest traps students fall into is "memorising" their pieces instead of reading them !! Build up your reading skills SLOWLY. Remember, notes in the first fret must be played with the first finger, notes in the second fret must be played with the second finger etc, play the "A" on the first string with either the 3rd or 4th finger.