It is expected that your writing is your writing. Plagiarism refers to the action of taking the words or ideas of another person and using those words or ideas like they are yours. This is viewed differently in different cultures. In some cultures, copying what another person wrote is a way to honor the original writer. In American educational settings, plagiarism is not viewed this way. Plagiarism in the United States is viewed as stealing another person's work. There are very serious consequences for plagiarizing. You should never plagiarize any part of any assignment in any of your classes.
There are many ways to plagiarize. You should be familiar with them so that you don't do it accidentally. Some examples of plagiarism include copying text word-for-word (or with a few changes) from something without citing the author, copying too much from one source, and improperly crediting the source.
Typically at this level, your writing will not require sources*, so this book will not instruct you on proper citation methods. You should ensure that the ideas you include in your essays are your ideas and that you have not copied anything from anyone. Make sure you choose topics for your essays that do not require you to search for information on the internet.
*Summaries (or the TOEFL integrated task) are an exception. You will practice giving proper credit through introductory phrases. You should use phrases that show the source of what you are summarizing like "according to the professor..." or "the reading stated that..." This is not the same type of citation that is required for academic essays.