turn over a new leaf

(verb) to begin to act in a better way

Example Sentences:

A: Do you have any New Year’s Resolutions?
B: Yeah, I want to turn over a new leaf. I’m going to start exercising more, and stop eating bad food!

The alcoholic man turned over a new leaf when he decided to live without alcohol.

Teacher: Wow, you’re doing your homework, little Dustin?
Dustin: Yeah, teacher! I’ve decided I want to become an accountant, so I’m turning over a new leaf. I’m going to start doing my homework every night!
Teacher: I’m proud of you.