When Charles Sizemore was a kid, he used to watch his grandfather furiously jot down stock tickers — and from then on, he was hooked.

Decades later, he now has his CFA, a master’s degree from the London School of Economics, is the founder and editor of the Sizemore Investment Letter, and manages client accounts through his firm Sizemore Capital Management.

Yet even he has made his share of mistakes along the way.

Charles says investors need to find a style that works for them — something that can only be achieved with a little experimentation. But even as you’re testing out things like trading, dividend investing and so on, you should be sure to have the core of your portfolio invested reasonably.

That’s because one of the most common investing mistakes — as Charles explains in the video above — is being overconfident. He says, “When you’re young, you think you are so smart and anything you pick will turn to gold. It’s not true.”

Check out the full video above to learn more about Charles, his own personal investing style, and what he says is the single-most important piece of investing advice for new kids on the block.

Like what you see? Sign up for our Young Investors e-letter and get practical investing advice delivered to your inbox every week!