How do I make sure I don't plagiarize on my paper?
Plagiarism is using someone else's ideas or words as your own. See the library's webpage on academic honesty and avoiding plagiarism http://library.uhd.edu/plagiarism.First, if you say something that's "common knowledge" you do not have to cite a source. So, if you said "Austin is the capital of Texas" you wouldn't have to cite it. But sometimes this can be tricky, if you're writing about a new topic and aren't really sure what should be considered "common knowledge". If you're not sure, you can always cite something and you'll be safe. Better to cite something and not need to, than not to cite it when you should have.
As for quoting a few words, and whether that's enough to need to cite it, that would be a judgment call. I'd ask you, are those words you would have chosen on your own? Are they describing an idea you already knew about? Are they a phrase that's commonly used by many authors to describe the concept? If so, you're probably safe not citing them. But, if you got that idea just because you read about it in that source, you'd want to cite it, to give credit to the person who provided you that information.
It's always ok to cite something. But it's often NOT ok to NOT cite something!