Newton's first law is interpreted as: A body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it. I was told by my teacher that a body has only two natural states i.e. state of rest and state of uniform motion. If viewed from the same frame of reference what caused the uniform motion in the first place as opposed to a body in rest?
Newton's first law describes how bodies move in the absence of an external force. As soon as external forces are involved, you have to turn to his second law $F=ma$, which describes how a force $F$ can cause an acceleration (and therefore an increase in velocity) of a certain object.
Hope that helps!