The first answer provides you with excellent advice. Here are a couple of other things to keep in mind.
Most schools that use turnitin.com also possess banks of their own students' work which doesn't always appear on sites which supposedly check your paper for plagiarism before you submit it. Thus, you will have spent money and time and possibly your integrity for naught if you have plagiarized anything from previous students' papers.
Secondly, many of my students are guilty of unintentional plagiarism. They paraphrase or summarize something but fail to cite it, or they put together a works cited page and then don't include parenthetical documentation even though we've spent weeks on how to cite correctly. The answer to that is simple--follow your teacher's--usually very detailed--instructions. Check your work and then ask someone else (a peer, another teacher, a parent, an older sibling) to check your work against your teacher's assignment description or rubric. Many times a pair of fresh eyes can catch plagiarism and stylistic problems that you have overlooked.
If you can learn now what plagiarism is (in its many forms) and how to avoid intentional or unintentional plagiarism, your life will be so much easier in college. For some universities, plagiarism is an automatic expulsion offense, and for others, it's enough for you to fail a course.
I am one of those "picky and strict" teachers, and I have a few suggestions for you. Plagiarism is a serious offense, as it is literally stealing ideas and/or words from another without giving due credit. The authors you used for research deserve the proper credit. Both solutions come at a cost, but at least you'll be correct.
Suggestion one is not the best solution because it does cost and there is no intrinsic learning involved; however, there are two sites which offer the service you seem to be looking for. One is mydropbox.com and the other is ithenticate.com. I've never used them personally, but I have heard of them by reputation.
Suggestion two is better in every way but it will cost you some time and effort. My students always know I will be checking every citation in their papers against the original source. Every one. No secrets, no surprises. The students who were most concerned about getting it right actually did what they knew I was going to do--went back and checked each paraphrase and direct quote against the originals. Most of the time they got it right, but many had inadvertently cited the wrong source or the incorrect page number, had failed to use quotation marks for direct quotes, had gotten too close to the original wording and needed to re-paraphrase, and probably several other kinds of errors. It takes some time and effort (thus the "cost"), but at least you'll be sure you're right as far as you know when you submit your paper. Plus, it's a good habit to develop if you're planning to attend college.