Possible causes include:

Jamming your toes by making sudden stops, especially while playing sports like tennis
or basketball.
Wearing tight-fitting shoes.
Clipping toenails too far back, so that the corners penetrate the skin as they grow out.
Being born with wider-than-average toenails.

Home remedies for a painful ingrown toenail include these steps.

Soak your foot in hot, soapy water three to four times a day.
Gently lift the nail away from the reddened skin at the outer corners, with the tip of a nail file.
Place a small piece of cotton soaked in an antiseptic just under the outer comers, if possible.
Repeat the previous three steps daily until the nail begins to grow correctly and pressure is
relieved. (Wear roomy shoes during this period.)

If the toenail edges become red and tender, fill with pus, or otherwise appear infected, see a doctor.

If home treatment fails to work, a physician or podiatrist may have to surgically remove the troublesome portion of the nail, or possibly the entire nail.

To prevent ingrown toenails, cut nails straight across. Don't cut the nails shorter at the sides than in the middle.

Note: Anyone who has diabetes or circulatory problems needs to be very careful to avoid infections of the feet. Trim nails carefully, or have them trimmed by a medical professional, to avoid nicking the skin or fostering an ingrown nail.