。
I'm ready to set out tomorrow.
Literal
Tomorrow go-out [possessive-の] preparation [subject-が] is-done-[state-ている].
準備ができている — literally 'preparation has come-into-being and persists' — means 'is prepared' as an ongoing state. Subtle aspect: not できた ('got done') but できている ('is in the done state'). Present-time readiness projected for a future action.