Newton's 1st law of motion: 'A body will continue to be in its state of rest or motion unless an unbalance external force acts upon it.'
Let a body move with an velocity 'v' having mass 'm'.
so it will gain a momentum due to its motion,which will be equal to product of its mass and velocity.
Now let an external unbalanced force act upon it.This will change it momentum.
Mass never changes ,so change must be in velocity.
As we know,rate of change of momentum is equal to force and rate of change of velocity gives acceleration.
This is the 2nd law of motion.