If you want to use the writing of another person, it is fine as long as you quote it. Quotations, however, need not be excessively used, unless you are criticizing the writing of another person and need the quotations to back up your arguments.
If you need to convey the message of a passage from the source material, you need to paraphrase. Paraphrasing means putting into your own words the meaning of some text. When you are paraphrasing from the source material, however, provide proper attribution. For example, you can write, "According to Dr. Emmanuel Sanchez, this disease can be cured completely in its early stages." You can then tell the reader what remedies Dr. Sanchez recommends to employ.
Summarizing is similar to paraphrasing. However, unlike paraphrasing, only the main meaning of the source material is conveyed to the reader. As with paraphrasing, summarizing requires diligent attribution: you need to clearly state where the information came from.
After you have completed a writing based on someone else's work, it is always a good idea to check whether you have been too close to the source. As most of the information can be accessed through the Internet these days, there are many plagiarism-checking online applications that look for occurrence of words to test whether a certain text is plagiarized or not. However, the plagiarism-detecting applications do not give a guarantee that a certain work is plagiarized or original. All they say is that a given text appears to be very similar to another text and human attention must be given to it. Popular free plagiarism-checkers include CopyTracker, eTBLAST, Chimpsky, Plagiarism-Detect and Plagium. The commercial options include Copyscape, Plagiarismscanner, Turnitin and Plagiarismdetect.