While the book we use, Modern Mandarin, has a dictionary of all pinyin we use in the back of the textbook, the following online resources may be helpful as well.
1. nciku.com - Excellent dictionary where you can save any words you look up in a list, as well as being able to trace a character with your mouse (for when we get to writing characters)
2. zhongwen.com - interesting web site that provides roots and philology of the written character. Make sure you click on "search" at the top of the page in order to look up a specific word.
If you want to purchase your own personal dictionary, I would recommend the Oxford Beginner's Chinese Dictionary. We will have a dictionary usage lesson when we begin using characters, but it is not necessary for you to go out and buy one at this point.