Why do my teachers tell me I can't use Wikipedia?
It isn't because it's online or that it isn't a credible source. Encyclopedias in general should only be used for background research. Find bits and pieces of information using the encyclopedia, then search for more information using what you learned.
For example, you'll find in an encyclopedia that the Iroquois lived in the long houses. So, rather than just search generally about the Iroquois, search using what you already know, try searching "long houses" to get more detailed information.