How this millennial millionaire manages 15 credit cards and his No. 1 piece of advice for beginners

Todd Baldwin has enough credit card airline miles to travel round trip from Seattle to Europe for free. The 28-year-old millennial millionaire loves to open new credit cards, especially those with sign-up bonuses, airline miles and cash back.

But Baldwin, who today estimates that he opens about two new cards per year, did not start out as a credit card optimizer. Growing up, Baldwin was raised by a single mom who worked multiple jobs to feed three kids. Seeing this, he felt motivated to make sure he didn't have to ever worry about money.

"That's how I got the fire under me because I knew I didn't want the struggle when I was older," Baldwin tells CNBC Select.

In his younger years, Baldwin associated credit cards with debt. But he quickly learned how useful they can be after he got his first credit card (a secured card) during his sophomore year at Western Washington University.

His number-one piece of advice to young adults is to start building a credit history as early as possible. It will come in handy when you want to apply for loans or take out a mortgage in the future.

"I didn't know that there was a good side to credit cards until I started researching more," Baldwin says. When you charge expenses onto your credit card, pay them off in full by the due date. That way you avoid any interest charges while also working to improve your credit score.

Promising himself that he would never charge more than he could pay off, Baldwin created a set of guidelines to follow when using his credit cards for everyday purchases, travel and more. Here is how he manages his 15 credit cards.

Baldwin may have 15 credit cards in his name, but he keeps only one in his wallet and the rest in a safe at home.

He notes that the card in his wallet is typically a flat-rate cash-back card that pays 1.5% to 2% on every purchase. He recommends these type of rewards credit cards, or one that comes with an interest-free period, such as a 0% APR credit card.

For those looking to have both, the Citi Double Cash® Card offers a 0% APR intro period for the first 18 months on balance transfers (17.49% - 27.49% variable after; N/A on purchases) there is an intro balance transfer fee of 3% of each transfer (minimum $5) completed within the first 4 months of account opening. After that, your fee will be 5% of each transfer (minimum $5), as well as 2% on eligible purchases with unlimited 1% cash back when you buy, plus an additional 1% as you pay for those purchases. Plus, earn 5% total cash back on hotel, car rentals and attractions booked with Citi Travel.

Even though Baldwin stores most of his credit cards away in a safe, he never closes any of them — especially his oldest one.

Closing a credit card can cause a temporary dip in your credit score because you lose the available credit limit on that account and your overall credit utilization rate increases. Since a good, or low, utilization rate entails having low credit card balances and high credit limits, a closed credit card makes your utilization go up and your score is affected.

Instead, keep your credit cards open and active. Letting them go dormant may signal to the issuer that you are no longer using the card and they could close the account on you. To make sure you are using your credit cards regularly, even if they are stored away in a safe, charge a small recurring cost on them, such as a monthly subscription or streaming service, that you automate payment for each month.

Baldwin knows that making on-time bill payments is the most important factor in achieving a good credit score, so he makes sure he always pays his credit card bills by the due date.

To ensure this, Baldwin automates his monthly payments so that the money goes out of his bank account at the same time every month, and he doesn't have to think twice about remembering to do it himself.

In fact, when it comes to paying bills, such as utilities on the six rental properties he has, Baldwin puts as many bills as he can onto one credit card and then automates that credit card payment every month to help his credit score. For those interested in the Citi Double Cash® Card mentioned above, following Baldwin's guideline would also mean you put money into your pocket since you earn cash back when you pay your credit card bill.

Carrying a balance on your credit card to boost your credit score is a common myth. When you have a revolving balance each month on your credit card, you rack up daily interest charges and eat up your available credit.

To prevent ever paying interest, Baldwin pays off any of his credit card balances in full every month.

Though some of the best credit cards come with annual fees, Baldwin has only one that he pays a yearly fee for.

His advice is to avoid cards with annual fees unless the card's benefits outweigh what you must pay to have the card. For example, Baldwin uses his only annual fee card, an airline credit card, to book travel because it comes with a companion fare that offsets the cost of the annual fee.

To help decide if a new card is worth its cost, calculate the amount of money you'd need to spend annually on the card to break even on the annual fee.

"It's important to never go out of your way to spend $1,000 in order to earn $300 because then you lose $700," Baldwin says.

Though Baldwin likes to take advantage of credit cards' generous sign-up bonus offers, he doesn't spend money he otherwise wouldn't just to get points, miles or cash back. Instead, he waits until he has a big purchase to make anyway before signing up for a credit card to receive the welcome bonus. Knowing that he has expenses coming up, whether it be furnishing one of his rental properties to list on Airbnb or making a repair on his car, he is more likely to reap new card bonuses in a way that works with his budget.

Because Baldwin pays his bills on time and never spends more than he can pay off on his credit cards, he doesn't worry about where his credit score stands. However, he does check his score for free every couple months, especially before he applies for a mortgage on a new real estate investment.

In fact, when Baldwin is in the process of getting approved to buy real estate he will avoid applying for a new credit card altogether. He knows that the hard inquiry that results when the card issuer pulls his credit report immediately dings his score temporarily and he wants to have the best possible score to show for when lenders check his credit.

To check his credit score, Baldwin uses the Credit Karma app, but there are plenty of resources available for consumers to access their credit score.

You don't have to be a Capital One cardholder to use CreditWise® from Capital One, which provides your VantageScore for free from TransUnion, one of the three main credit bureaus.

To take a deeper look at his overall credit and what makes up his 3-digit credit score number, Baldwin says that he pulls his credit report every 12 to 18 months and/or before he buys new property.

Consider checking your credit report for free at AnnualCreditReport.com.