？
Are you ready for tomorrow?
Literal
Tomorrow [possessive-の] preparation [topic-は] could-be-done?
できた is the plain past of できる ('to be able/to be done/to come into being'). With 準備 it means 'preparation is done/has come together.' The rising intonation (？) turns the statement into a yes/no question. Very casual — a parent or friend check-in.