Be honest. Be ethical. Don't be selfish.
Memorize this: The problem of plagiarism occurs when you fail to give credit for information, such as ideas and words, that are not originally yours.
In academic writing any ideas or language not credited to another are assumed to be that of the author.
It's simple: If you use someone else's words or ideas and you don't tell the reader where you got your information, the reader will logically assume the words and ideas are your original work. It's misleading and dishonest.
Of course, in college, the reader is typically your instructor. And instructors take a very dim view on cheating. They're also smart enough to find out.
Let's put a positive spin on it: Suppose your reader wants to follow up on an idea you've written about. How will your reader know where to look if you don't tell him or her? That's just selfish.
The problem of plagiarism occurs when you fail to give credit for information, such as ideas and words, that are not originally yours.