The guitar I will be referring to in the following sections is an acoustic 6-string guitar. Your left hand is then the picking hand, which rests on the neck of the guitar. The six strings are pressed onto the fretboard by the player's hand, which shortens the part of the string allowed to vibrate when plucked, which changes the pitch of the string. The result of this is that the two notes sound the same except that the second one has a higher pitch. This will make more sense as the tutorial progresses.
Music works in a very mathematical fashion.
The way that these waves interact with each other lead us to what we call music. The most important thing to understand is the fact that there are 12 tones and they repeat after the 12th tone. The study of musical theory is a vast and expansive subject matter, the purpose of this is to give you enough background to continue the tutorial. A and B are examples of a whole step. Be careful when figuring whole steps versus half steps.