Here's how to pay for things with your smartphone

Most of us leave the house carrying three essentials: keys, wallet, and smartphone. But over the past few years, more and more people are combining the latter two objects. No, we're not talking about phone cases that also hold cash. Your smartphone can store your financial details and use them to make secure, instant payments in the store. It may feel vaguely futuristic, but you can check out with a wave of your mobile—as long as the place you're shopping has the necessary hardware.

If you're new to the world of app-based payments, but want to upgrade from the old cash and cards approach, it's easier than you might think to get set up. In fact, you have most of what you'll need already installed on your phone. Here are your payment options.

Apple Pay first saw the light of day in October 2014. Now it comes pre-installed on most of Apple's hardware products, including the Apple Watch and the iPhone (at least on version 6 or later). You can use this program to pay for purchases online, within apps, and in physical stores: Any outlet that supports contactless payments, such as Best Buy, Staples, the Disney Store, Starbucks, Walgreens, and many more, will accept Apple Pay.

To set up Apple Pay on an iPhone, open Settings, then choose Wallet & Apple Pay. From here, you can link your credit or debit cards to the phone's payment system. It also lets you add new cards and edit the details from existing ones.

To add the same card or cards to your Apple Watch, use the Watch app. Go to the My Watch tab and tap on the Wallet & Apple Pay option. This will give your wearable the same paying power as the card you've added, which means you'll be able to pay with your smart watch even when your iPhone isn't nearby.

Once you enter your card details, you're ready to pay for goods. To do this in a physical store, you can use any contactless terminal, even if it doesn't display an Apple Pay sign (though the sign is worth looking out for). As long as the checkout takes contactless payments, it should work with Apple Pay as well. Unlock your phone with a passcode or Touch ID, to prove it's actually you using it, then tap your iPhone (or Apple Watch) on the terminal to pay.

This system works works through a technology called Near Field Communication, or NFC. Your card details aren't actually beamed from your phone to the terminal. Your device simply confirms that it's authorized to act on your behalf, using an encrypted code it received when you registered your card with it the first time.

Because this system is unfamiliar, it may breed security concerns. But Apple Pay can actually protect your money better than a card. While someone who steals your credit card would then be able to make multiple payments with it, that's not possible with an iPhone—they would need the phone's Touch ID or passcode to access your earnings.

Android Pay launched in September 2015, a year after Apple's payment system, and it works in a very similar way. Its primary purpose is to let you quickly pay at the checkout in a retail store, but the tech has also found its way to apps and online shopping. Android Pay works on phones and tablets using Android 4.4 or higher, as well as Android Wear watches (such as the LG Watch Sport) that include both NFC and Android Wear 2.0.

You can manage all of your cards and other payment information through the Android Pay app on your phone. If you already have registered certain cards with Google (for in-app payments and the like), then you can quickly add these to Android Pay with just a few taps. On an Android Wear watch, you will need to install the Android Pay app separately, then use it to select one of the cards you've previously placed on your phone.

Like with Apple Pay, you need to have some sort of lock on your phone (like a fingerprint or passcode) to use Android Pay, as this can verify your identity. If you're using a smartwatch, you don't need to unlock your phone every time you pay. However, if you take your watch off, then you will need to reauthorize the connection (by unlocking your phone) the next time you use Android Pay in a physical store.

Android Pay should work with any terminal or checkout that supports contactless payments, though you can also look out for the Android Pay badge. Outlets from Macy's to Whole Foods have signed up for the system. In the store, make sure your phone is unlocked, tap it against the terminal for a few seconds, and wait for the green light. Again, the system will send an authorized token rather than your actual card details—just one more way that Apple and Android Pay resemble each other.

Samsung Pay, despite the name, works on any modern Android phone, not just Samsung devices. You can also install it on the Gear S2 and Gear S3 smartwatches. Once you've added the Samsung Pay app to your device, add your cards within the app. When it comes time to pay, simply unlock your phone (which can even be done via an iris scan on a Galaxy S8), and then hold your phone against the terminal to use your card.

Samsung Pay has one big advantage over Apple Pay and Android Pay: It works with both contactless terminals and those terminals that use the old magnetic stripe technology. In the latter case, however, you do need a recent Samsung device, because the method requires extra in-phone hardware. That ability makes Samsung Pay compatible with a broader range of point-of-sale terminals.

We've covered your options if you want to use your phone in a physical store. But what if you just want to pay back your friend for dinner, or settle the cable bill with your roommates? In this case, you've still got plenty of app options. Like with Apple, Android, or Samsung Pay, you will need to link these apps to a real bank account or card. But the process is usually pretty painless.

PayPal (Android, iOS) is perhaps the best-known and one of the longest-running mobile payment options. Recently, its apps for phones have caught up to its desktop site in terms of ease-of-use and functionality. If you link a bank account to your PayPal, or accumulate a PayPal balance, you can pay anyone (at least, anyone else who has a PayPal account) for free. Although you can also link a debit or credit card, using this payment method will incur a fee of $0.30 plus 2.9 percent of the total amount.

In the world of payment apps, Square Cash (Android, iOS) is the up-and-coming contender, looking to significantly improve the way we all send money to each other. Perhaps its best feature is that the person you're paying doesn't need their own Square Cash account: You can use the app to pay anyone whose email address you know. Square Cash lets you link debit cards for free, but adds a 3 percent charge for linked credit cards.

For an even smoother experience than either PayPal or Square Cash, try Venmo (Android, iOS), which PayPal actually owns. This socially-connected option is built more like a messaging app than a payments tool. It even includes an option for splitting a bill. Payments are free from a linked bank account, major bank debit cards, or a Venmo balance. Otherwise, you face a 3 percent charge for credit cards. Like with PayPal, the person you're paying must have a Venmo account as well.