Ok. This lesson is really important.
It's for beginner or for people that don't know how to tune their guitar.
First, the small key on your guitar : That's what you will move to tune your guitar.
Here's a excercice to learn the sound of your guitar. The 6th string at 5th fret is the same note has the 5th string at 0.
Try to play this and look if the sound match.