To avoid plagiarizing, you must change both the sentence structure and the words of the original text.
How to paraphrase
Read the original text until you grasp its meaning; then set it aside.
Using your memory, write down the main points or concepts. Do not copy the text verbatim.
When reading a passage, try first to understand it as a whole, rather than pausing to write down specific ideas or phrases.
Be selective. Unless your assignment is to do a formal or "literal" paraphrase, you usually don't need to paraphrase an entire passage; instead, choose and summarize the material that helps you make a point in your paper.
Think of what "your own words" would be if you were telling someone who's unfamiliar with your subject (your mother, your brother, a friend) what the original source said.
Remember that you can use direct quotations of phrases from the original within your paraphrase, and that you don't need to change or put quotation marks around shared language or common vocabulary shared by a community of scholars.
Check your notes against the original to ensure you have not accidentally plagiarized.