Newtons first law of motion doesnt define force. It defines inertia - the tendency of matter to resist changes in motion (change of speed or direction). The more the mass, the more it resists changes in its motion. This resistance is called inertia.

A force is something that will change an objects motion (speed or direction) if it is not balanced by another force. Put another way, a force that is not balanced causes an acceleration (a change in motion). Forces are measured in Newtons or in pounds, and have a direction, so we say a force is a vector (a quantity that has a magnitude and direction).