Some things you do not have to cite! These include:
Your opinion
It is yours after all. However, you will want to cite sources that support your opinion.
Things you observe and report
You see your best friend kissing someone special and describe how they looked, you do not have to cite it as the observation is something you witnessed and reported.
Things you experience and report
Experiencing something first hand makes for a much more vivid experience that just observing it. Your own experience with a first kiss is uniquely yours and if you write about it that is something that cannot be attributed to anyone else.
Things you create
If you write a poem about that person your first kissed that is your own work and does not need to be cited. Similarly if you do a research survey on first kisses the data your gather and report are yours and do not have to be cited.
Common Knowledge
Quick, who was the third president of the United States? You may not know it was Thomas Jefferson, right of the top of your head, but any reference source will tell you the same thing. If there is no significant disagreement with factual knowledge, you do not have to cite it.
Accepted Factual Knowledge
The earth is shaped like a bulging sphere. With the exception of a few flat-earthers, this is generally accepted fact and does not need to be cited. Writing a persuasive paper on the earth being flat, on the other hand, would require significant citing as this is not accepted factual knowledge.
The concept of plagiarism finds its origins in theft. According to The Oxford English Dictionary (OED), the word plagiarism appeared in the in the English language in the early 1600s.
A related word, "plagiary," according to the OED, means a kidnapper!
Using exact wording
If you use someone else's content word-for-word, put it in quotation marks and cite it. Direct quoting should be kept to a minimum. Your professor (and future employers or clients) want you to be able to express your own idea clearly, not just cut and paste.
Using someone else's ideas, conclusions, etc.
Even if you do not take some one's idea word-for-word, you still much cite that source as something you borrowed from.
Stuff that is hard to cite.
Some sources are difficult to cite as they do not seem to fit any format. Even though it is difficult, cite it as best you can. Your professor or the Writing Center tutors can help you with this.