According to Newton's first law, what is inertia?

Inertia is an object's tendency to resist changes in its state of motion.

A bowling ball and a beach ball are the same size. Which has more inertia and why?

The bowling ball has more inertia because it has more mass.

Net force is the vector sum of all forces acting on an object.

Equilibrium is when the net force on an object is zero, resulting in no acceleration.

I can use Newton's laws to describe and predict the motion of objects.

1
Newton's first law describes inertia, which is an object's tendency to resist changes in its state of motion.
2
Newton's second law states that net force is equal to mass times acceleration, or F = ma.
3
Newton's third law states that for every action, there is an equal and opposite reaction, which explains the normal force.