You do NOT have to cite anything that is COMMON KNOWLEDGE: Stuff that everyone knows without having to look it up or ask someone what it means or research the topic.
You MUST cite anything else. Truly. If it is not something that just everyone knows, YOU MUST CITE THE SOURCE OF THAT INFORMATION!!!
The Online Writing Lab from Purdue University will be able to answer every, single question you might have about your citations and much more. I use it myself, almost daily, to make certain that I am properly citing, avoiding plagiarism, and all sorts of things having to do with writing. Just follow the link and scroll down a bit to MLA GUIDE > MLA Formatting and Style Guide >Then Choose the type of Citation you need.
EasyBib is a great tool that can create your citations for you. However, as with any tool, you MUST check what it produces to see that it has given you correct citations. Don't assume anything is right until you check it!
EASYBIB ADD-ON for Google Docs, makes it seamless to use.