It is a good idea to have a smartphone application which helps you tune your guitar. There are zillions of free options there. Just install and follow the instructions. Or just use “guitar tuner” keywords on Google and you will find lots of them.
Option number 2 is to buy a dedicated guitar tuner. Go for it if you have a budget – it’s worth!
If you have neither software or hardware tuner, then you go with the “classic” way to tune:
You need a reference note for your 1st string – High E (the thinnest one). Take it from internet or some other instrument like piano (this is best). Tune your string with the machine head, spinning it round until you find the right tonal spot.
Now that you have 1st string tuned you can go further. Place your finger on the 5th fret of the second string and play the note. Then play 1st open string. Adjust the second string to be equal to the 1st. Listen carefully and make it as perfect as you can. Both notes should have exactly the same pitch. Do the same thing with 3rd string, 4th fret vs. 2nd open string. Then go with 4th string, 5th fret vs. 3rd open. Next is 5th string, 5th fret compared to 4th open. And the last one – 6th string, 5th fret vs. open 5th.
Do it with high attention to details as all string pitches depend on each other. Every little mistake could cost you going back to where you first did it incorrectly.