Knowing how to tune your guitar is essential for achieving a good sound. The biggest reason for this is that as beginners you are teaching not only your fingers to find their way around the guitar fretboard, but you are also teaching your ear to recognise chords and notes. When you tune your guitar, your guitar is in tune and your ear is learning the correct notes; when you do not tune your guitar properly, your ears get used to the sound of slightly sharp (or very sharp) or flat notes. This is very bad for developing your musicianship and strengthening your ear training. The diagrams below shows indicates what the open notes are.
I’m getting a lot of messages from people coming here because they want to learn to play guitar as part of worship. If this is you, you might want to join Aaron Anastasi’s programme. Aaron is a musician and a worship leader. He put together a step by step programme for aspiring Christian musicians. If you're interested, you can use the exercises and advice in learn-classical-guitar-today to develop your finger strength and dexterity and Aaron's lessons to work on strumming and worship music.
There are many android and IS apps that work perfectly well as metronomes and there are a few that market themselves as guitar tuners. Other than sometimes having an irritating beep, there are many android and IS apps that function just fine as metronomes. I can't say the same of the android and IS apps that are supposed to work as guitar tuners. I have found them sketchy and very frustrating to use as they are just not that consistent. Nonetheless, if you can't afford a tuner or a metronome, these virtually free android and IS apps are good options to get you going and to get you started.
In the absence of an electronic tuner you can use a pitch fork or pipes or a piano to tune your guitar. Tune your bottom E string (the 6th string is also the thickest and lowest soundest string).
Place your first finger behind the 5th fret of the 6th string (E). Pick this note. You are now playing A which is the same note as the A on the 5th string which is sounded by playing the 5th string open. Keep your finger on the A (6th string) and at the same time play the A (5th string). Gently adjust the 5th string until it is in tune with the 6th string.
Repeat the above process to tune the 4th string, the D string. Place your first finger behind the 5th fret of the 5th string (A). Pick this note. You are now playing D which is the same note as the D on the 4th string which is sounded by playing the 4th string open. Keep your finger on the D (5th string) and at the same time play the D (4th string). Gently adjust the 4th string until it is in tune with the 5th string. tun
Repeat the above process to tune the 3rd string, the G string. Place your first finger behind the 5th fret of the 4th string (D). Pick this note. You are now playing G which is the same note as the G on the 3rd string which is sounded by playing the 3rd string open. Keep your finger on the G (4th string) and at the same time play the G (3rd string). Gently adjust the 3rd string until it is in tune with the 4th string.
Repeat the above process to tune the 2nd string, the B string, except that now you are playing the B in the fourth rather than the fifth fret of the 3rd string. Place your first finger behind the 4th fret of the 3rd string (G). Pick this note. You are now playing B which is the same note as the B on the 2nd string which is sounded by playing the 2nd string open. Keep your finger on the B (3rd string) and at the same time play the B (2nd string). Gently adjust the 2nd string until it is in tune with the 3rd string.
Repeat the above process to tune the 1st string, the E string. Place your first finger behind the 5th fret of the 2nd string (B). Pick this note. You are now playing E which is the same note as the E on the Ist string which is sounded by playing the Ist string open. Keep your finger on the E (2nd string) and at the same time play the E (Ist string). Gently adjust the Ist string until it is in tune with the 2nd string.
This success of this approach is partly determined by how developed your ear pitch is. If you have any doubts about your this, please purchase an electronic guitar tuner. They are affordable and will help you develop the security of ear pitch that you require. If you want to get something more expensive a very nice tuner, with metronome combined, that I can recommend is the Korg TM-40.
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You'll need to go a little deeper into music theory as you proceed. The best online resource for this is Guitar Theory Revolution. It recognises that music theory is very hard for guitarists because music theory has till now been located in the piano paradigm. The Guitar Theory Revolution overthrows the piano paradigm that is holding you back and embraces the attributes of the guitar to unpack music theory. In fact, it goes further and allows you to see that the guitar is one of the best instruments for learning theory.