Difficulty: Moderately Easy
Step1
Whether you're buying or renting gear, you want to find the right gear or you won't be able to enjoy your new experience. All equipment--masks especially--is something that you need to see, touch and test before purchasing. Avoid buying this type of equipment over the Internet.
Step2
Mask. A good mask is one you put on your face and it creates a seal. To see of there is a seal, suck in with your nose. If air seeps through anywhere around the perimeter of the mask, you don't have a seal. If you can't get any air in your nose, you have a seal and the mask fits. This insures that when you're in the water, no water will seep into the mask.
Step3
Snorkel. Snorkels are easy. Some have mechanisms built in that will help you purge the water when some gets inside, but these are extra features and more expensive. A good snorkel will fit well on your mask when it is on your head.
Step4
Fins. If you don't plan on swimming far, smaller fins are fine. If you plan on swimming some distance, get larger fins. A good fin will fit your foot and be comfortable barefoot or with wet boots on. Try on the fin and make sure it doesn't rub anywhere. This could cause blisters.