Some forces are always the same-

Weight is always downward..

Normal reaction perpendicular to the body

On a plane, force due to wt along plane is mgsin@

"        "         "               "     perpendicular 2 plane is mgcos@

Tension is always along the string away from the body..

Friction on a body is always opp the direction of motion of the same body.

and so on..

 

First thing is to identify the system.Then,determine what all forces act on that system.

If it is at rest,then its simple..forces act in their usual directions.

If its moving,then the net force acts in the direction opp motion of the body.