Carefully examine the student's work to determine whether or not you can discern their intention. Many students plagiarize unintentionally and need a little direction. Although it is ultimately their responsibility for reviewing college or university regulations regarding plagiarism, some students simply "slip through the cracks" and get into college with a poor research and writing background. They may even read the rules regarding plagiarism and not fully understand what the rules say. Try to distinguish between intentional and non-intentional plagiarism. Intentional plagiarism usually consists of material not "quoted" by the student when copying the source in a verbatim fashion, and the student may even leave the source material out of their bibliography or works cited page.
Create a paper trail. A verbal confrontation with a student will never suffice when it comes to issues of academic integrity and related discipline. Your first correspondence with the student should be an email or a letter that can be printed and placed in a file for later use if needed.
Provide the student with an opportunity to explain the transgression. In your email or letter, let the student know that it "appears" as if there may be some similarity between their work and someone else's. Avoid using the word "plagiarism" initially as it is too confrontational. Let the student know that you would like to discuss the matter further, which you can then do either in person, on the phone or via written correspondence. Given the chance, many students will come clean if they know they've been caught. Of course, some students will try to wriggle their way out of the situation, but the burden of proof is on them since you have their work as evidence.
Give the student a choice. Those who have committed a non-intentional plagiarism transgression should be given the opportunity to redo their work the right way. Do so with the understanding that repeat instances will result in further and more severe disciplinary action in accordance with college or university regulations. Handle intentional instances of plagiarism more harshly. A zero for the assignment or for the course should be enough of a setback for most plagiarizers, but some universities will use expulsion as a disciplinary measure.