To snorkel you don't need a lot of complicated equipment—a mask, snorkel, fins, and swimsuit or swimming trunks are all you need.
Mask. It's important to buy a good mask that's the right size for your face and has a tight-fitting seal. The easiest way to test how well your mask fits is to lift the strap over the top of the mask and press the mask to your face without breathing in. If it stays tight to your face (without you holding it on), you've got the right fit. If not, keep looking until you find one that seals properly. You'll want to get a mask that has a glass face (not plastic) to keep it from fogging up or making the sites below look weird.
Snorkel. There are many different sizes and designs of snorkels—find one that is comfortable, and allows you to clear water easily. It's important that you find the right snorkel for you—it's what helps you breathe while you're cruising along in the water checking out the sites below.
Fins. There are a few different types of fins to choose from—full-foot fins fit like a slipper around your heel, while open-heel fins fit your feet and have a strap that fits around your heel. Your fins should be snug, but not too tight (if your fins are too tight or loose, they may cause blisters). They should be flexible and lightweight—to give you speed and mobility. You may also want to get diving booties to prevent blisters and protect your feet.
If you're swimming in salty water, make sure to rinse all of your equipment with fresh water. If you don't, salt crystals can form causing the straps to stiffen and crack, and the fabric may tear. So, keep your equipment clean. Remember, it's what allows you to move through the water and check out the underwater world more easily!