1. Force Diagrams (Free-body Diagrams)
A force diagram is simply a diagram showing all the forces acting on an object, the force's direction and its magnitude
It is a simplification of the picture that shows just the forces. 

2. Steps for drawing a force diagram:
1. Identify the object you will draw a diagram for.
2. Identify all the forces acting directly on the object and the object exerting them
3. Draw a dot to represent the object of interest.
4. Draw a vector to represent each force.  Draw it in the direction the force is being exerted, and label it by (a) the type of force, (b) the object exerting the force, and (c) the object receiving the force (which will be you object of interest). 
5. If the object is stationary or is moving at a constant velocity, the vectors should graphically add up to zero.  If the object is accelerating, the sum of the vectors should produce a vector in the same direction as the acceleration.

3. Types of forces typically used
Symbol
Magnitude
Direction
1.Gravity (weight)
F(G)
 = mobject*g        (9.8 m/s2)
Downward
2.Normal (surface)
F(N)
any (up to breaking load)
perpendicular to surface
3.Tension 
F(T)
along string/rope/chain
4.Friction
F(fr)
not slipping:  between zero and ms*F(N) slipping: = mk*F(N)
Direction opposing motion.

4. Writing down the sum of the forces
1. Identify direction of every force and of acceleration.
2. Pick a coordinate system to minimize the number of things (forces and acceleration) that must be broken into components, especially unknown values
3. Draw the components for any forces or acceleration that does not lie along the X or Y axis, and identify the angle that is given (or being looked for).
4. Pick one direction and write down all the forces or components of forces in that direction, using positive and negative signs to identify those in the positive and negative directions.
5. Set the sum of the forces in that direction as equal to the mass multiplied by the acceleration in that direction.  (If not moving or moving at a constant velocity in that direction, acceleration will be zero.)
6. Repeat for the other direction.

6. Websites
Free-body diagrams
Newton’s Laws
More Of Newton
More free Body diagrams