To ‘Turn over a New Leaf’ means to start over, to act in a different manner or change your attitude about something.
Example of use: “After years of drifting from job to job, Danny turned over a new leaf and got himself a steady gig.”
Way back in the 16th century, pages in books were referred to as leaves. So tuning over a new leaf meant that one was turning to a blank page. This idiom was used to signify a major change in behavior, or a new stage in life.