Guitars will need to be tuned continuously to ensure that it is tuned for the correct scale and notes so your music makes sounds the way you intend. If you haven't been playing forever, you will probably need the assistance of an electric tuner to find the note. This video will show you how to tune your guitar using a tuner.
This is a guide for the people that don't have digital tuner and for the people that have one.
Turn the tuner to the "on" position. Face the guitar in close proximity of the tuner and play the first note (low E).
Check to ensure the tuner is receiving the signal. Turn the tuning peg on the guitar higher or lower in order to alter the pitch of the instrument.
Without touching any of the strings on your fretboard, cleanly, but gently pluck the top E string with your guitar pick.
The Boss TU2 digital tuner.
If using a digital tuner, the guitar string will be in tune when the LED of the digital display is indicated by a green light. It will indicate whether the note is either flat or sharp by a red light. If the light moves beyond the center of the intended note (E in this case), it is considered sharp and needs to be tuned down to get to the center. If it doesn't quite reach the center, then it will need to be tuned up.
When using an analog tuner, the same method is applied, only you will be dealing with a meter (needle) instead of the cool LED. The center or "in-tune" area of the meter is at "0."
Repeat this process for all subsequent strings, remembering to follow note E with A, D, G, B, E and going from top to bottom; lowest to highest.