。
What should I do if the rainy season starts tomorrow?
Literal
By-chance tomorrow rainy-season [into-に] enter-[conditional-たら] how do-[volitional-よう]?
梅雨 ('rainy season,' literally 'plum rain,' from the plum ripening time) is Japan's roughly month-long rainy period in early summer. 梅雨に入る is the set phrase for 'the rainy season begins.' どうしよう ('what shall I do') is a volitional interrogative expressing distress or uncertainty.