In my humble opinion; if you learn to read for guitar only, the best thing to do is learn tablature (or tab). Guitar is different from piano in that a note can be played in several different places on a fretboard, versus only one place on a keyboard. Therefore there are different ways to play the same piece on a guitar. Tablature tells you (suggests) which fret to play the note on. Also tab is relatively simple to learn, it’s more visual, and has less rules than regular written music. I don’t know of any tablature tutorial sites, but I haven’t looked either.