Yeah, and Michael Jordan can hit a fadeaway jump shot better than you can, and Miles Davis could play a better blues than you do on the trumpet. Learning to write is learning to think. Sure you won't have a lot of original thoughts, very few of us do. But you will have your original way of looking at things, which is a combination of everything you have done to this point in your life. As you read others' works and ponder, argue with, distill, reconcile yourself to, or reject them, you are growing intellectually, just as you would grow physically by lifting weights or playing the piano.
I thought I can use someone's words if I reference or cite the source.
You can, and this happens all the time in academia. It is necessary for building upon the works of others. The trouble comes when you start to use someone else's words all throughout your paper. Pretty soon your paper looks like nothing but a field of quotation marks with a few country roads in between (your few sentences) connecting them. This does not represent very much intellectual work on your part. You have assembled a paper rather than writing one.
Some people set out to deliberately plagiarize, but I am not talking about them. I am talking about how you will get yourself into trouble by adopting the vocabulary words and phrases of an author, using them throughout your paper, and not thinking that you have to put quotation marks around each phrase or key word.
Consider the following passage from Heilbroner, Robert L. An Inquiry into The Human Prospect. New York: W.W.Norton & Company, 1974, page 37:
The race between food and mouths is perhaps the most dramatic and most highly publicized aspect of the population problem, but is not necessarily the most immediately threatening. For the torrent of human growth imposes intolerable social strains on the economically backward regions, as well as hideous costs on their individual citizens. Among these social strains the most frightening is that of urban disorganization. Rapidly increasing populations in the rural areas of technologically static societies create unemployable surpluses of manpower that stream into the cities in search of work. In the underdeveloped world generally, cities are therefore growing at rates that cause them to double in ten years--in some cases in as little as six years. In many such cities unemployment has already reached levels of 25 percent, and it will inevitably rise as the city populace swells. The cesspool of Calcutta thus becomes more and more the image of urban degradation toward which the dynamics of population growth are pushing the poorest lands.
There are a number of characteristic phrases here that say a lot in just a few words: "the race between food and mouths," "the population problem," "torrent of human growth," "urban disorganization," and "cesspool of Calcutta." If you use these phrases in your paper without indicating that Heilbroner wrote them, or just put one reference to Heilbroner at the end of the paper in the bibliography, you are committing plagiarism, even though you thought you were just trying to express your ideas better than you otherwise could.
For example, if you wrote something like this:
The cesspool of Calcutta is a good example of a city where urban disorganization is being threatened because of a rapidly increasing population. These cities are technologically static, economically backward, and impose intolerable social strains and hideous costs on their individual citizens.
and put no reference to Heilbroner, it would certainly be plagiarism. In the Honor Council pamphlet, this kind of plagiarism is depicted in Example IV and is called "The Mosaic." If you put one reference to Heilbroner in the bibliography, it would still be plagiarism, because your citation was too vague. You could write it this way:
The "cesspool of Calcutta" is a good example of a city where "urban disorganization" is being threatened because of a rapidly increasing population. These cities are "technologically static," "economically backward," and impose intolerable social strains and "hideous costs on their individual citizens." (Heilbroner, 37)
You have lived up to the letter of the law--you have given the proper citation. But have you fulfilled your job as a writer? One could argue that the reader would be better off reading Heilbroner's original paragraph. All the "meat" in your paragraph is his. It depends on how this paragraph fits into the larger whole of what you are writing, of course, but this may be a case of not doing enough intellectual work yourself.
By the way, did you know that even using one of these small, characteristic phrases without quotation marks is considered plagiarism? This is called "The Apt Phrase" and is presented in Example V in the Honor Council pamphlet.
What should you do? Really think about what you are reading, outline an argument that reflects the conclusions you are drawing, and when you flesh out the argument, write mainly in your own words, adding quotations where they are necessary to acknowledge others' thoughts and to present evidence that helps your argument. Obviously if you are working at the last minute, you won't have time to do this. It doesn't matter if you are using the Encyclopedia or looking at your roommate's paper for the same course: if it's not your work, cite it.