Here's a link for you:
The answer depends on things like where you live, type of house, what type of ventilation your attic currently has, and if you use the space as living space. Can you give us more info? it would be easy to say get some batt insulation and throw it on the floor, but that may not be what your house needs. Your local utility company probably has an energy saving program where they send someone to your house free of charge to do an energy audit and then make recommendations. They may also offer insulation services and materials at a reduced cost that is subsidized by the utility company.
Here is a helpful video: https://www.youtube.com/watch?v=fB6CbJsb1FM
Many kinds of insulation: Fiberglass rolls, resin panels, blow-in and foam. A lot depends on the climate you live-in and they type of attic you have. Some utility companies have an annual analysis of your home for free to make recommendations to improve your heat retention and lower power use. They use a thermal camera to show you where you're losing energy.
Stores like home depot have blow-in insulation and if you buy so many bags they will give you the machine to use for free/cheap.
If you are just doing over the floor, fiberglass rolls can be used between the rafters with a staple gun.
The foam is usually done by pros.