You have to know the material densities for the materials used e.g. Concrete, bricks plaster water proofing. Next you have to consider the loading on the floors.The loading can be calculated as load due to dead weights and loads due to live loads.
To calculate load due to roof slab, find the area of the roof slab, multiply by thickness and density of the material used for roofing. This can be done for a unit area say per sq. m/foot and density in KN/cu.m or foot as per your units of working . To this add load due to waterproofing treatment considered e.g. BB coba or membrane. Consider live load as per specifications and category of the building. Don't forget to add load due to parapet wall if any.
Similarly calculate the load due to dead load of roof beams if provided in case of RCC structure. This can be calculated by using l*b*h*density of concrete. Calculate for all columns individually. To this add the load from slab based on area supported by that column
Calculate load due to brick wall inclusive of plaster by using the same formula. The wall thickness for internal walls and external walls may be different. This load may be transferred to the column through plinth beams if provided. In case of single storey building the wall can be supported adequately on ground also. Take your call. The load through columns is then transferred to foundation. By using simple mathematics and calculator this can be accomplished