1) In the absence of external forces a body that is at rest will remain at rest, a body that is moving will continue to move at constant speed in a straight line.
2) When a total force F acts on a body of constant mass m the body will accelerate at the rate a, in the direction of F, where F=ma.
3) If body A exerts a force F on body B then body B exerts an equal and opposite force, -F, on body A.
Force is a vector quantity so we can add and subtract forces using the usual rules of vector addition. The dimensions of force are kg m s-2 and the unit is called the Newton. 1N=1kg m s-2.
When two bodies are in contact we can split the force between them into two perpendicular components. We call the component perpendicular to the surface the Normal Force and we call the force parallel to the surface Friction.
When a body of mass m is situated in a gravitational field of acceleration g it feels a force, mg, which we call the Weight of the object.
Any time that we are considering a problem involving two or more bodies interacting with each other and the environment we should analyse the situation with a set of Free Body Diagrams. A free body diagram shows a single isolated body and all the forces acting UPON it. The vector sum of those forces must be equal to ma for that body. It is important to include only the forces that other things exert ON the body and not forces that it exerts on others.