Dictionary.com is a good place to start your research. There is also Etymonline.com, which gives you the history of words in alphabetical order. Oxford Dictionaries also has answers to some questions regarding the etymology of certain words.
Etymonline is especially good because of how well organized the website layout is. For example, if you choose to look for words starting with the letter A, all the words beginning with A are in alphabetical order. You can click through the pages beginning with A, until you find the one you were looking for. There are 60 pages worth of A words.