I do recommend you Word Reference, you can download it as well and it is a quite smart app. You can listen to the pronunciation, it keeps you search history and you 've got access to the forum. http://www.wordreference.com/

Diccionario de la Real Academia Española (DRAE)

My professor has told us that we need to invest in a good Spanish Dictionary (book) that has Spanish word and Spanish definitions. Having the English Translation would be a good bonus.