Turnitin is an assessment tool used by your tutors, but you can use it as a tool too - to check that you have good Academic Integrity.
What does this mean?
How do I submit an assignment to Turnitin?
For instructions, see your tutor.
This is a help guide on interpreting your results once you've already made a submission.
The truth is, there is no "perfect percentage".
Any piece of work will have some of the same words as other assignments, or have used the same sources as other assignments, so you will always have a percentage of matching words.
Here's some other reasons why your percentage might be higher than you expected:
In general, none of the above are reasons to be concerned if you are following academic integrity guidelines.
HOWEVER, if you are concerned, talk to your tutor.
But wait. My percentage is REALLY high. What now?
Check the following reasons for a high percentage - there might be an easy answer:
Turnitin Similarity Report Demo - YouTube
The University of Portsmouth have created a helpful video showing you how to interpret your results once you've submitted an assignment to Turnitin. It is full of trips and tricks to get a more realistic similarity percentage (so that the percentage number on your report is not so scary). It uses an older version of Turnitin (The Similarity Report is called The Originality Report in the video), but the functionality is the same.