Final answer:
The adverb 'first' in the sentence modifies the verb 'picked', indicating when Ginny Weasley picked her costume.
Step-by-step explanation:
The bold word 'first' is an adverb. In the sentence 'Ginny Weasley picked her costume first.
'the adverb 'first' is describing when Ginny Weasley picked her costume.
Therefore, the adverb 'first' modifies the verb picked.