In order to design footings of a building, you need the loads from the above structure and also you need to know the soil conditions such as the soil type, bearing capacity etc. then you can first do a preliminary design with rough dimensions etc. to see if it checks out, and then proceed to the final design. You should have a structural engineer do all these and definitely have your design stamped as this is a safety item. Then you will need to submit this to the city for approval