A brand new car looks and smells good — but it's never worth the price, says self-made millionaire and bestselling author David Bach.

"Nothing you will do in your lifetime, realistically, will waste more money than buying a new car," he tells CNBC Make It. "It's the single worst financial decision millennials will ever make."

That's because the moment you drive it off the lot, the vehicle starts to depreciate: Your car's value typically decreases 20 to 30 percent by the end of the first year and, in five years, it can lose 60 percent or more of its initial value.

To make matters worse, "most people borrow money to buy that car," says Bach. "Why would you borrow money to buy an asset that immediately goes down in value by 30 percent?"

The good news is, you can get a shiny, nice-smelling car without breaking the bank, Bach says: "Buy a car that's coming off of a two- to three-year lease, because that car is almost brand new and you can buy it at that 30 percent discount."