、。
If it rains tomorrow, we'll go by bus.
Literal
By-chance tomorrow rain [subject-が] fell-[conditional-たら], we [topic-は] bus [with-で] go [informative-よ].
~たら is slightly more colloquial and conversational than the more formal ~ば — both work for 'if it rains tomorrow,' but ~たら fits casual speech better. Final よ informs the listener of the plan — the speaker assumes the listener doesn't already know what's been decided.