Real Academia Española (RAE) - The Royal Spanish Academy is Spain's official royal institution with a mission to ensure the stability of the Spanish language. This is a tool that establishes the criteria for its correct and proper use in order to contribute to the splendor of the Spanish language. You can use RAE when in doubt as to which is the correct/proper Spanish word to be used. 

Duolingo - This is a great way to review and practice what you have learned! You can download the free app on your phone or use the web page.