Write a system of two equations. First, the sum of all forces acting on the beam is 0. The forces are: the weight of the load, the beam's weight applied to the middle of the beam (these two are directed down), and two supporting forces at C and D (these are directed up).

The second equation says that the sum of torque (moment) is 0. Torque is calculated with respect to some pivot point, but in statics one can choose pivot point arbitrarily. So you can choose, e.g., the center of the beam or its left end. Then the torques of some of the four forces will try to rotate the beam clockwise and others counterclockwise; those torques come with opposite signs.