Final answer:
The adverb 'first' in the sentence 'Ginny Weasley picked her costume first' modifies the verb 'picked,' indicating the order of the action.
Step-by-step explanation:
In the sentence 'Ginny Weasley picked her costume first,' the adverb 'first' is used to describe the verb 'picked.' It tells us the order in which the action took place, indicating that Ginny was the earliest to choose her costume, compared to others. This aligns with the general function of adverbs, which is to modify verbs, adjectives, or other adverbs, often indicating the manner, place, time, or degree of the action or condition they are modifying.