Hi Dora Jimela , thanks for reaching out.
It is important to remember that the Turnitin Similarity Score represents the percentage of your students’ writing that is similar to something found on the internet, in our databases 
or in someone else’s paper. Similarity does not mean that this work is plagiarised. As the educator, consider a student’s use of quotations, citations and bibliographic material when investigating the Similarity Score. 
Generally, there is not a right or wrong number to receive as a score. However, certain scenarios can dictate whether you should expect more or less similarity from your students’ work.
I'd like to extend an invitation to attend a free training session on "Interpreting Turnitin's Similarity Report" which is repeatedly numerous times per month, at times to suit educators in PNG. Click here to view the latest schedule and register for a session of your choice. You're welcome to share this link with your colleagues as well.

Dora Jimela Kialo I also recommend the following resource: "Setting reasonable expectations for the Turnitin Similarity Score"
https://www.turnitin.com/papers/setting-reasonable-expectations-for-the-turnitin-similarity-score