Putting a bit of graphite or pencil lead in your nut can really help with tuning. The strings slide through the nut uninterrupted making for a more exact tuning. The nut is located up by the head of the guitar, it’s white and made of a petroleum product. It sets the height and space of the strings up at the top of the guitar. Note that when a nut is too high the strings can be harder to push down, this can slow your playing down and make for an uncomfortable playing experience. Ask your guitar teacher if you think your nut is too high and then contact a guitar luthier such as Mal at Weta Guitars in Owhiro Bay for the adjustment.