You may be tempted to quote passages from your source. If you enclose these passages in quotation marks and include a citation, then you will not be guilty of plagiarism. However, science writers generally don't quote because they are expected to use other scientist's results and ideas, not the original author's words. It is the ideas and results that are important, and we use these to lend credence to our ideas or to compare and contrast with our results. When we rely on quotes, we are not adding any knowledge to the scientific community.
There are three main reasons that students use quotes:

  1. they don't fully understand the passage they are reading, but think that something in it is important for understanding their study
  2. the passage they are reading uses a lot of fancy words or has a complex sentence structure that the student can't figure out how to paraphrase
  3. they feel that the quoted text best describes, in an eloquent way, the point that they are trying to make.

The solution to #1 is to determine how the information fits into your narrative. For example, does it help you explain why this study is important?
The solution to #2 is to remind yourself that in science simple writing is better because complex writing makes it harder to understand the scientific importance of what you are trying to say.
The solution to #3 is to describe your thoughts, hypotheses, or interpretations of the work you are citing rather than just presenting the original author's ideas.
One strategy that might help you with all these problems is to first explain, out loud, to someone unfamiliar with what you are writing what the passage you are using means. Then, explain how this information fits with your study. Once you can explain it out loud, you should find it easier to write it out. If you don't have a willing audience, try recording yourself!