Slab self-weight is thickness times density. Reinforced concrete is about 25 kN per cubic metre, so a 150 mm slab weighs about 3.75 kN per square metre. Here is how to get it right.
Whether a slab spans one way or two changes how load reaches the beams, and what shape the tributary areas take. Here is how to tell them apart and when the distinction actually changes your numbers.
A slab hands its load to the beams that support it, as a uniform strip in one-way action or as triangles and trapezoids in two-way action. Here is how, with the load shapes.