Tuning your guitar is a very easy thing to do, whenever you siblings play around with your guitar and dis-tune it, you should be glad because it gives you more experience with the guitar. You can use a piano to tune your guitar or you can do it with
your mind, that is if you are already very familiar with the sound of the notes of music. You could download an app on google play or in the apple app store for it.
So for begginers, i will advise you to use the piano.
If you look at the strings of the guitar, the strings have their respective notes when they are open. look at the diagram above. The first string has the E note when open. the second string has the B note when open, the third string has the G note when it is open, the fourth string has the D note when it is open, the fifth string has the A note when it is open and the sixth string has the E note.
On the piano
All you have to do is to play the respective notes on the piano and whine the knob of the string you want to tune until they make the same sounds. Be careful as you whine so the strings will not snap.