Reducing the similarity percentage? My only response to that is...don't plagiarize!
Find out exactly how your prof wants you to document your references.
Keep all drafts of your work. Always.
Cite all references. Always.
With respect to APA, if that is what you're required to use maybe these will help:
How to Cite References Using APA Style
APA Citation Style · Library "How To" Guides · Help & Instruction · Concordia Libraries
As a side note....
At the university I attended students could refuse to submit their work to turnitin.com, without any penalty.
If you want to know what your options are, ask for more information from your student union (or what ever group represents student concerns at your school).
Hope this helps:)
Not a teacher.
Student or Learner