The start of a new year is the perfect time to set professional plans and physical fitness goals, and it's also a great opportunity to make a commitment to better your mental health. But often, that's much easier said than done.

Mehmet Oz — famously known as Dr. Oz — has dished out health and wellness advice for nearly a decade on his talk show, "The Dr. Oz Show." And, in his expert opinion, the most beneficial thing you can do for your mental health in 2019 is simple and straightforward. The best part? It won't cost you a cent.

"The best thing you can do in 2019 to create the mental health you want and deserve is to be proactive in your life," Oz tells CNBC Make It. "Don't be seen as a passive player in someone else's story. Be the lead character in your own story. So, decide what you want to do, and just go after it. Doesn't mean you have to be abrasive or aggressive, it just means that you have to feel like you control your destiny."

The feeling of being able to control your own destiny can be harnessed by creating easy habits, he says. Those habits can include you deciding what time you wake up in the morning and when you go to sleep at night — which, Oz says, can have a profound impact on your mental health.

Sleep, he emphasizes, is the "No. 1 most underappreciated problem in America," particularly for younger people.

"Evolution would have weeded out sleeping if it wasn't an essential," Oz says. "Why would you be a species that could sleep eight hours a night when animals could eat you and take advantage of you? We sleep because we have to. It cleanses our brain. Literally, the brain cells shrink a little bit to allow the channels in between them to to freely clean out toxins to sort of reboot the brain every evening."