It is good practice to trim your nails on a regular basis considering that they grow on average about 1-2 mm per week.
This can be in the form of a regular trim where you are significantly reducing the size of the nails. It can also take the form of a light trim where you are simply smoothing and reshaping the edges of the nails.
For your regular trim you can use a nail clip. This should be sharp enough to cut through the nail, but not too sharp as to cause any damage. Make sure that when you are trimming the nail you do so in a fairly linear fashion especially for the toenails. By going too much in towards the flesh at the sides you run the risk of the nail edge growing into the flesh. This is called an ingrown nail which can lead to infection and is very painful. Dont forget that infants nail must also be trimmed regularly. They are sharp and infants will scratch.
Especially for the ladies the smoothing of the nail surfaces can be achieved with an emery board.
They come in all shapes and sizes, so you should be able to easily find one that suits your fingers and style. Use the emery board on the rough areas to smooth and shape them. Dont use it on the normal nail surfaces. It is recommended that after trimming you should not remove the cuticle as is commonly done. The cuticle helps to prevent infection spreading under the nail bed.
Good nail care is paramount especially for persons with certain conditions like diabetes or nail infections like onychomycosis. The latter is a fungal infection of the nails making them discolored, thick and brittle and is quite common among diabetics. It is important that diabetics keep their nails properly trimmed to prevent breaks and other cuts. When in doubt get a professional manicure/pedicure. If a problem develops seek advice from a podiatrist.