Siri’s default search engine is Microsoft Bing. There is no option to change the default to use other search engine such as Google or Yahoo!.
You however can say “Search Google for” to get Siri to use Google to search for the results you want. For example ask Siri to “Search Google for Captain America”, and Siri will launch Safari and display the search results for “Captain America”. Similarly you can say “Search Wikipedia for” to search for entry in Wikipedia.