Final answer:
The correct sentence is 'I played badly today'.
Step-by-step explanation:
The correct sentence to use in this situation is: I played badly today. In this case, 'badly' is the correct adverb to describe the way Lily played. 'Bad' is an adjective, which would be used to describe a noun. Since Lily is describing how she played, she needs to use the adverb form, which is 'badly'.
Learn more about Choosing the correct adverb to describe an action