The first thing you need to do before you start your search is to focus on what is being asked. Think about what you need to find by asking questions about your topic.
To find out about
"How has England's relationship with Australia changed since 1945?"
some relevant questions might be:
You may need to consult some background material, such as encyclopaedias or general works to help you. Your answers may depend on the context in which the question is being asked (e.g. are you studying History or Law?).