。
Dinner is ready.
Literal
Dinner-of preparation [subject-が] become-done-[polite-past-ました].
Note the verb here: 出来ました is the past polite of 出来る ('to come into being, to be done, to be ready'), not the potential 'can do.' 出来る in this sense describes the completion of something — cooking, homework, a task — as a natural process. 支度 ('preparation, arrangements') is a slightly old-fashioned word, more common than its synonym 準備 in domestic contexts.