Gingivitis is a condition of inflamed gingiva caused by plaque and poor oral hygiene. Good news, though is that it is easily treated with improved oral hygiene and by visiting a dentist.

[edit] Steps

Brush thoroughly twice a day. Do this every single day. Most importantly brush right before going to sleep. Brush properly and thoroughly for a minimum of two minutes, even the back side of the very back teeth.
Go see your dentist or hygienist every six months.
Use mouthwashes containing plaque fighting and anti-bacterial ingredients.
Use anti-cavity and anti-gingivitis toothpastes that contain fluoride.
Remember to floss twice a day,

[edit] Tips

Eat right to ensure proper nutrition for your jawbone and teeth.
Mild inflammation of the gums due to plaque buildup, red or swollen gums, gums that bleed easily, are all signs of gingivitis. If you notice any of these, see a dentist, or follow the guide.
Avoid cigarettes and other forms of tobacco
It usually develops during puberty or early adulthood due to hormonal changes and may persist or recur frequently depending on the oral hygiene status of each person. So don't worry if you are an adolescent and are having gingivitis, just treat it.