I'm not sure what you're talking about with "I'm doing my sats soon." SAT's?? Specific questions about specific pieces of literature are not on the SAT I.
However, to answer your question, Beatrice loves her cousin, Hero. When Claudio denounced her at the wedding, he ruined her reputation, and for a lady of that day and age, her reputation was everything. If Hero's name had not been cleared, she never would have been able to get married or have children. So, Beatrice wants to avenge her cousin's dishonor. She wishes she could challenge Claudio to a duel herself, but since she can't (as a woman), she asks Benedict to be her champion and challenge Claudio for her.