When the storm ended, we started getting ready to set sail.
Literal
Storm [subject-が] ended-when, we [topic-は] setting-sail [possessive-の] preparation [object-を] began.
Note that the subject of the subordinate clause (嵐) takes が, not は — a regular rule for Japanese: subjects inside subordinate clauses (relative clauses, とき clauses, quoted clauses) take が. The main-clause subject 私たち takes は normally. This pattern is worth internalizing early.