How to charge a Nikon d80?

My dad has one from like 6 years ago and I was wondering how to charge it since the battery is low

You need the charger that came with the camera. Your dad should have it.

If you don't have a charger, look at the battery model number.
Go to
http://www.bhphoto.com
Do a search for the battery. If you find it click on the Accessories tab. That may bring up a compatible charger.

Remove battery… Put battery in charger. Red LED will blink until battery is fully charged. Steady red means it's charged.