When quoting, be sure to quote word for word, down to the punctuation. Any omissions, additions, or modifications to the quote must be signaled to the reader with ellipses and square brackets, as explained in Advanced Quoting Skills.
While minor modifications are permissible, you must never present the quote in a manner that changes the author's original meaning. Especially if done with the purpose of distorting the author's intention and/or manufacturing support for an entirely different argument, intentionally changing the meaning of a quote is academically dishonest and risks severe penalty.
Be aware that your professor is likely to notice when a quote doesn't seem "right," and will probably double-check it. Indeed, many professors have their favorite examples of this dishonest practice. Here's one, taken from a student paper on immigration. The student was arguing (incorrectly) that most immigrants who came to America experienced economic success. Not surprisingly, she was having a hard time finding a quote to back up this claim. While some immigrants experienced economic success, of course, the reality is that as many immigrants (and indeed many more, in sheer numbers) did not.
The closest our student could find was the following quote from John Bodnar's book The Transplanted, a standard book on immigration to the United States:
"Some immigrants came to America and acquired large fortunes, and many more simply went to work everyday with no appreciable gain" (Bodnar xvi).
Our student deleted the first word along with the entire second part of the sentence and provided the following quote in support of her claim that most immigrants who came to America experienced economic success:
According to John Bodnar, "immigrants came to America and acquired large fortunes" (Bodnar xvi).
See what she's done? Her professor did, and marked down her paper accordingly. Be smart, be honest, quote accurately, and do not distort an author's intended meaning. If you are having a hard time finding quotes to back up your thesis, consider whether your thesis may be shaky. Rather than manufacturing false evidence, re-think your argument.