Plagiarism Tutorial by University of Texas Librarians. It has everything to help you avoid plagiarism and be in a good academic standing.
You should cite when:
You give statistics.
The information is unique and not known by most people.
The reader might ask, "How do you know that?"
You use a direct quotation from someone else.
You use someone else's ideas
You paraphrase a direct quotation from someone else.
You do not need to cite when:
The information is commonly known (either by the general population, or commonly known within the particular discipline).
It is your own original thought or opinion.
What if you are not sure?
If you are not sure, it is better to include a citation.
From: More Hand-On Information Literacy Activities by Fiona Hunt and Jane Birks.