Sign Convention for Shear Force (SF):

As the shear force is the unbalanced vertical force, so this tends to slide one part of the beam, upward or downward, w.r.t. the other as illustrated in Figure .

 Figure: Sign Convention for Shear Force (SF)

This is said to be positive, at a section, while the left hand part tends to slide upwards w.r.t. the right hand portion. Or, in other terms, all of the upward forces to the left of the section shall cause positive shear. This is said to be negative, at a section, while the left hand part tends to slide downwards with respect to the right hand part. Or in other term, all of the downward forces to the section shall cause negative shear.

Alternatively, all of the upward forces to the right of the section shall cause negative shear & those acting downwards shall cause positive shear.