How do you convert kg's to newtons?

You don't convert kilograms to newtons.  By the time you've heard of these units, you know that 'kilogram' is a unit of mass, 'newton' is a unit of force or weight, and that mass and weight are different things.

Mass and force are related by Newton's second law: 

                       Force = Mass x acceleration .

From this simple formula, you can see that in order to relate a mass to a force, you need to know an acceleration.  And if the acceleration changes, then the relationship between the force and the mass also changes.  So there's no direct conversion.

ON EARTH ONLY, one kilogram of mass weighs 9.8 newtons. The acceleration that connects them is the acceleration of gravity on Earth.  In other places, with different gravitational accelerations, 1 kilogram weighs more or less newtons.

But they don't convert directly.  That would be like asking "How do you convert miles to miles-per-hour ?" 

Multiply the 3 dimensions, assuming it is a rectangular cuboid, to get the volume in cm³ divide mass by volume to get density in g/cm³