In given case, our system look like
Forces involved are (i) weight of beam (ii) weight of mass placed over beam (ii) Reactions of supports which we have to found.
Free body diagram of our system is
In static equilibrium problem, calculation involves following equations
Sum of all vertical (or vertical component of) forces is zero,
Sum of all horizontal forces or horizontal components of forces is zero,
Sum of all torques is zero.
You may take signs as
All forces along + y-direction are positive.
All forces along + x-direction are positive.
All forces along - y-direction are negative.
All forces along — x-direction are negative.
All anti-clockwise torques are positive.
All clockwise torques are negative.
In given problem, as the beam is in equilibrium