It is extremely important that you understand the difference between an online dictionary and an online translator. Not understanding the distinction could land you in hot water with the Academic Honesty Policy!
The online dictionary that I recommend is Wordreference.com. There is a link to it on the Sakai menu.
Dictionaries only allow you to type a word and give you a definition that should look much like a definition given in a printed dictionary (i.e., shows the part of speech and multiple translations, based on context of use.) If you can type phrases in English and the device or website gives you a Spanish phrase, it is a translator and cannot be used.
The following video shows you how to use the Wordreference.com site to find the best word for what you're trying to say.
Using this site is always ok, but if you choose to use a different one and are unsure about whether it's acceptable or not, please just ask - before completing the assignment.