How to question how do i know what kind of rafters to put in a ceiling that will be walked on?

builiding into the garage adding more space in my living room i know how to build walls but the roof/ceiling is becoming a question of unanswered questions want to be able to put storage on top of the ceiling so how do i support the wieght of boxes plus my self. And also i have a 20ft ceiling in garage and dropping 10ft to add the storage what kind of bracings do i need