If you do not have an account on internationally approved plagiarism programs such as Turnitin, use one of the other available programs (eg, Plagiarism Checker) where you will be given "approximate" values for similarity. Try to have the similarity value away from the specified value.
1) Try to minimize the use of internet sources in your papers. 2) Try to accurately cite all the sources in the paper. 3) Use one of the online services such as Turnitin.com to check the similarity index.