So you have a guitar, and you can hear the music, but do you actually know what notes you are playing when you pluck the strings? There are plenty of great musicians who can play the instrument, and just hear what sounds good, but knowing the notes will help you progress in your song writing. It will help you establish what key you are in, and lead you to the next note that completes the measure.
The first thing you need to know are the six basic letters used in music: A B C D E F G. To move from one letter to the next in music, you use what are called whole steps and half steps. To move from a natural A to a natural B is a whole step. To move from a natural B to a natural C is a half step. In the natural form of the letters moving from a B to C and an E to F are the only half steps in the sequence. The rest are whole steps. Even from a G back to an A is a whole step. If you moved a half step up from A then you would be on A sharp or B flat. This note is the same, but the name of the note depends on what key you are in. This is the same for C to D, and F to G.
Now how do you relate this to your guitar? In standard tuning, your guitar is tuned from top (lowest sounding note) to bottom (highest sounding note) as E A D G B E. More advanced musicians will use other tunings for their guitar, but this is the basic standard tuning. You also need to understand the frets on the neck. The frets are the lines that are on the neck of the guitar, and you typically hold down the fret in the spaces in between the bars, but as close to the next bar as possible. The first fret would be the first space closest to the head stock (where you turn the knobs to tune your guitar). Once you get this concept down, you can start fretting down different strings on your guitar and finding the other notes.
Each fret is considered a half step. To move a whole step, you have to move up the neck two frets. Now practice walking up the first string E. Remember that between E and F are a half step, so if you fret E on the first fret and pluck the string, then you are now playing the F note. To get to natural G on same string you will move up two frets (the third fret) because it is a whole step to get from F to G. To get back to A move up two more frets (you should now be on the first string, fifth fret). To go to B, you will move up two more frets (the seventh fret). Now to get to C, you will only move up one fret, because it is only a half step from B to C. So the first string, eighth fret will be the natural C note. Then two more frets to get to D (the tenth fret). Then move up two more frets and you will be back to E, but an octave higher (the twelfth fret). A little short cut is the twelfth fret of each string is the same note as the string played open, just an octave higher.
The same concept is applied to each string just with a different note. Remember the second string is the A string. To get from a natural A to a Natural B is a whole step, so this time you will fret the second string on the second fret. This is the natural B. You will follow the same patterns of whole and half steps up the neck on each different strings to get to the notes that you want. Once you understand the whole and half steps of each natural note, you can throw in sharps and flats of each note by only moving one fret (a half step). For example if you fret the second string A on the first fret, you are now playing A sharp or B flat. Again the note is the same. You name it according to the key that you are playing in. Go through each string and find the natural note first, and then start playing with the sharps and flats.
Once you have gone through the string finding the natural notes from the bottom to the top of the neck, you can find each note at the bottom of the neck, just moving down the strings. Remember the top string is E when plucked open. Then fret it on the first fret and you are playing F. Then up two frets (the third fret) and you are playing G. Now move down to the second string open on A. Then fret the second string on the second fret and you are on B. Then move up one fret to play C. Then move down to the third string open and you are playing D. Then fret the third string on the second fret and you are playing E. Then move up one fret to play F. Then move down to the fourth string open to get to G. Fret the fourth string on the second fret for A. Then move down to the fifth string open for B. Fret the fifth string on the first fret for C. Then move up two frets (the third fret) for D. Then move back down to the sixth string open, and you are back to E. This will be the same sounding note as the first string open on E, but two octaves higher. You have just gone through all the natural notes from E to E on all strings in the first position in two octaves!
Now you can figure out what any note is that you are playing anywhere on the guitar. Practice saying the note in your head while you are playing, and this will help with memorizing the notes and where they are. You can find the same sounding note in several places on the guitar. Just play around with the different strings and different frets. Music is a form of art, and knowing the notes are another tool to make you that much better!