
There are literally dozens of different methods that teachers employ when teaching their pupils how to play guitar, and there is not necessarily a right or wrong method. When I was learning guitar, however, I already had a background playing the piano, which made the process much easier. As a matter of fact, most music teachers suggest learning to play the piano before embarking on another musical endeavor.
The reason why the piano is good as a starting point is that it is unparalleled at training people how to read music. Due to the layout of the keys, it makes learning to read and play music easier. While some musicians claim to never read or write music, they are the exception to the rule, and a good grounding in musical notation is beneficial to any aspiring musician.