Guitar basically has six strings. Although  some guitar has seven, eight or more strings. But six string guitars are tuned to E standard (440 Hz) EADGBE from top to bottom strings.
6th string   = E (82 Hz)
5th string   = A (110 Hz)
4th string   = D (146 Hz)
3rd string   = G (196 Hz)
2nd string  = B (246 Hz)
1st string   = E (329 Hz)  
Guitar strings are tuned in such a way that every string repeats after fifth string. That’s mean if first string is E then the sixth string will be another E, but of another octave. Same happens with seven string guitar. In seven string guitar the second string is B, so the seventh string is also B. This pattern follows whatever the number of strings. 


How to tune the guitar?

Tuning guitar is hard especially if you do by ear. Here are some methods to tune.
1. By piano/ keyboard:
We can tune with the help of piano. To tune, play E note (frequency equal to high E) and turn your tuning peg of first string to match the frequency. Again play the B note on piano and turn tuning peg of second string. Play G note on piano and adjust the frequency of third string of guitar. So on you can tune the whole guitar. 
2. By ear:
To tune the guitar by ear, we need a reference note or frequency. You can tune high E string with the help of piano, keyboard or tuning fork etc. 
Once you tuned the first string, you can find the same note (E) on fifth fret of second string. You play fifth fret of second string and compare the frequencies. You turn the tuning peg of second string and match the frequency of open E to 5th fret second string. Once you matched the frequency your second string is in tune. And you do same procedure with third string. But notice that open B string has to be matched with B note on 3rd string, but now the B note is at fourth fret of third string. After tuning third string, now its turn to tune fourth string. Open third string is G and you can find G note on 5th fret of D string. Once you tune D string do same procedure with A string. You can find D note on fifth fret of A string. And you can find A note on fifth fret of low E string. That’s how you can tune the whole guitar. 
3. By tuning fork:
Tuning with tuning fork is similar as tuning with ear. Here, tuning fork provides the one reference frequency or note. The tuning fork has a only frequency and you tune your guitar with that frequency. As we know we find same notes on different strings, we can tune the whole instrument as tuning by ear. But tuning with tuning fork is not common nowadays because it is very hard and we have many other options available. 
4. By electric tuner:
It is the easy method. You can tune your guitar very fast by the help of electric tuner. You can find built-in tuner in some guitar. It is very easy to use. Once you turned it on, it starts showing whether the guitar is in tune. You play the open note and it shows whether it is in tune or not. If it is in tune it shows the name of the note and turns to green, and if it is not in tune it remains red. By doing same procedure, you can tune your whole guitar.
5. By tuner app:
It is also very easy method. The tuning procedure in it is same as an electric tuner. But being app, it is very easy to have it. Since most of the tuning apps in play store or apple store are free, you can easily download it and tune your guitar.