All you need for snorkeling is a snorkel -- a special breathing tube that allows you to swim face-down in the water without lifting your head to breathe -- and a mask. While the inexpensive masks available in drugstores might work for you, you will have a better experience if you try on a few masks to find the one that feels comfortable, offers a tight seal and affords good visibility. A poor-fitting mask will ruin your swim by forcing you to stop every few minutes to clear water from the mask and tighten the strap.
Choose several masks within your age range -- child, youth or adult -- to try on. Adults with small, narrow faces should try youth-sized and adult-sized masks.
Hold your breath and place the first mask gently against your face. Do not inhale or exhale. If the mask feels comfortable, go on to the next step. If it feels uncomfortable, remove it and try another mask.
Press the mask gently against your face and let go. Do not inhale or exhale. If the mask falls off your face, it probably won't seal properly so try another mask. If it doesn't fall off, move on to the next step.
Hold the mask on your face with one hand and pull the strap over your head with your other hand. Tighten or loosen it slightly if necessary to keep the mask from sliding. If you need to tighten the strap more than slightly, choose another mask.
Test visibility by looking around without moving your head. Choose the mask that gives you the best peripheral vision and feels the most comfortable. If you wear glasses or contacts, you will enjoy snorkeling more if you wear contacts under your mask or invest in a mask with corrective lenses.