Learning to tune your guitar should be one of the first lessons of any guitar training or teaching program, including learning the guitar string’s order, tuning, and how to keep the strings in tune. It’s important to learn early on for several reasons:
-An in-tune guitar sound far better than an out of tune guitar!
-The sooner you figure out how to tune your guitar on your own, the faster it becomes a habit to do so.
-Especially when playing with other musicians or your instructor, being out of tune draws attention to yourself (and not the attention you want!)
-If you’re out of tune, even hitting the right notes will make you sound like you’re playing incorrectly.
Would you go for a walk without tying your shoes? You could, but it’d be silly and for some, even risky.
If you’re beginning guitar, consider the practice of tuning a necessity. Just like tying your shoes, it’ll take a bit of getting used to, but soon it’ll become second nature. The sooner you make it a habit, the better, as it just happens to be an important practice. There are few things more unpleasant, and certainly discouraging, than thinking you’re playing the right notes without them sounding right.
How to Tune Your Guitar
1) Learn your guitar string’s order, illustrated to the right.
For the sake of clarification, this image is as if you’re looking straight at the guitar. If you were holding the guitar as if you were playing it, the thickest string on the guitar will be closest to you, and the strings will become thinner as they approach your lap.
The numbers will come in handy later, but for tuning purposes aren’t altogether necessary.
The best way to remember them that I’ve come across (or a “mnemonic”, which is a memory aid) is remember (thickest to thinnest string)
(E)ddie (A)te (D)ynamite, (G)ood (B)ye (E)ddie
Pretty clever, huh? It’s not original, I’m afraid.
2) Learn the appropriate “pitch” of the strings.
Pitch simply refers to identifying if the string is a “low” note or “high” note.
For example, your “low” E string (the closest string to you as you hold the guitar) needs to sound more like, say, Morgan Freeman while your “high” E string should sound more like Justin Bieber. (Sorry, I couldn’t resist.)
Every string in between your low E and your high E will increase in pitch, or put another way, should get higher in pitch.
3) Bringing each string “up to pitch”. 
If you’ve messed around with your tuning machines (aka: tuners, tuning keys, tuner thingies, etc) you’ve probably noticed that turning them one way or the other changes the pitch, and subsequently makes the string tighten or loosen up. Obvious as it may be to some, I’ll say it anyway. The tighter the string is, the higher the pitch. The looser the string, the lower the pitch.
As a matter of trivial knowledge, each string, once up to the appropriate pitch could be measured according to pounds of pressure which is generally between 14-17 pounds per string on most guitars. Why is this important? Because when you get into guitar maintenance, this will explain a lot about why guitar necks undergo some pretty funky changes- assuming the average amount of tension per string is about 16 lbs, 16 x 6=96 lbs total pressure. This weight is CONSTANTLY being applied to the guitar neck. But more on that later.
Once up to pitch, using the online guitar tuner here, watching a YouTube video, using a piano to tune, or whatever, you are now good to go.
Common Tuning Problems
Having changed well over 3,000 individual guitar strings in the last 5 or so years, I’ve noticed some common things when coming across tuning issues.
99% OF GUITAR TUNING ISSUES ARE A RESULT OF THE GUITAR BEING STRUNG IMPROPERLY
How is this best remedied? Well, proper stringing technique will be covered in other writings. If you keep falling out of tune, tug gently 2-3 times on each string, bring it up to pitch (back into tune), and repeat one or two more times. If this doesn’t correct the problem, your strings may be poor quality or simply defective. Drop by your local guitar store and get some assistance if you think you fall in this population.
Other possible explanations may include:
-Slipping tuning keys (tuners, tuning machines, etc). This is what many people end up suspecting, and unnecessarily spending money to replace when the fact is that the above action could much more cheaply solve their problem. These days, even generic tuning machines are pretty reliable. There are obviously exceptions, but in most cases, even cheap guitars have decent enough tuners to keep the guitar in tune.
-Defective, soft guitar neck. This will invariably cause the strings to go flat (or loosen, bring them below pitch). In one or two cases, I’ve seen guitar with hairline fractures (broken necks) that bring the guitar out of tune.
-Cheap strings. As you might expect, this is also very common. There’s no need to spend $12 on strings every single time, but anything by D’Addario or Ernie Ball will prevent this from being your problem without shelling out more than $6-9 for a set. Many guitar shops will even put them on for a small additional fee of $10-20. Guitars with special bridge systems (namely Floyd Rose bridges) will often cost between $20-30, as the process is considerably more involved. On the other hand, if you’re being asked to pay any more than this by an establishment, you might consider going somewhere else. (They’re asking too much)