Nails to an advanced classical guitarist are like a bow to a cellist. They need to be taken care of and you cannot play without them. However, if you have no nails or cannot grow them for sports reasons etc., don’t worry. In the beginning it is probably more important to have a solid technique than nails. If you do have nails and wish to use them, you will need to know how to file them correctly.
Nails differ from finger to finger and from person to person, so this is just a guide. Each nail should be about 3-4 mm long, as in the following photos. The longest nail is usually the ring finger. Each nail should be carefully filed so that it has no sharp edges or corners. Rather than filing straight on, file the nail from underneath so that it thins out gradually towards the tip. The shape of the nail should not be too pointy nor completely straight, because the idea is to play from the left of the nail in towards the middle. Once happy with the shape, rub your nails along a treble string and listen for any catching or scraping sounds. These can be smoothed out with a superfine emery board.