I need a concrete footing to be 5 feet tall. What circumference and reinforcement do I need?
when you are building things 24 feet up and asking these kind of questions, it should be self evident you need to hire someone to do it for you or at the very least handle the technical design and ensure that what gets built is inspected and checked to make sure it matches the design specifications. 

do yourself and any of your future guests and neighbours a favour, and get an architect and a general contractor involved.