user defined data type

hi

my aim is to put a the UPDATE operation result showing which field updated with having details of field name, old value & new vale.
i had written the trigger for it, and that working fine.
But for a very large database i want to use array for dynamic length.
I want to create user defined data type and want to used in array type for input the output of my trigger of UPDATE operation.

how can i built user defined data type?
how can i used my user defined data type for array?

with regrds
vikas