Onchange event of Select field how can we call ajax and populate another field

hi

i am using cobalt 8 and have a problem. i am using cobalt fields and those fields are dependent on each other e.g field 1 is categories (cricket,football,hockey etc) and field 2 is subcategories. if i select football from field 1 i want to populate field 2 by subcategories of football

now my question is that how can i call ajax by onchange event of select (categories) and populate other field dependent on first field

regards