After you are done stringing the guitar, you should stretch out the strings a little bit. Different people use different methods for stretching strings. I usually put the strings on my guitar, tighten them up a little, and then bend all of the strings up and down, bending them towards both sides of the fretboard. Some people pinch a string near the bridge and pull it away from the guitar, while some people with a whammy bar will just five it a couple tugs and a few dive-bombs to warm up the strings.
The purpose of stretching the strings is so that they will stay in tune once you start playing. If you string up your guitar and then tune it without stretching the strings, it is guaranteed to go out of tune after the first song that you play. You should stretch the strings as much as you can before you start playing in order to avoid having to retune.
One thing that many guitar players never learn to do properly, but is very important, is tuning the guitar. In order to completely understand the concept, you need to know the pitch of each string.
In standard tuning the strings run, from thickest to thinnest: E, A, D, G, B, and E. If you have an electronic tuner, all you have to do is plug in your guitar and tune each string to its corresponding pitch. If you have an acoustic guitar, and there is no built-in microphone on your tuner, then just plug a microphone into your tuner and play your acoustic through that.
Using a tuner is fine, but there will be a time and a place where you will need to tune a guitar, and there will not be a tuner handy. In such a case, you will need to know the relationships between the tunings of the strings
On the low E string, the fifth fret has the same pitch as the open A string. On the A, the fifth fret is the same as the open D. On the D, the fifth fret is the same as the open G. The fourth fret of the G string is the same as the open B, and the fifth fret on the B string is the same as the open high E.
These relationships are very important. If you can grasp the concept of the way the guitar is tuned, it will make tuning and playing it a whole lot easier.
