The First Law states, "A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force."
This simply means that things cannot start, stop or change direction all by themselves. It requires some force acting on them from the outside to cause such a change.
The most famous equation is
Fnet= Mass * acceleration
This means that the Net force is equal to the mass of the object times its acceleration
