Snorkels are designed to allow you to breathe at the surface of the water with your face submerged. They are also used extensively in free diving. The need for a snorkel while diving has come into question in recent years because they create drag, are snag hazards, and can really get in your way while diving.
However, you were or will be trained in your scuba class to use and carry a snorkel while diving. There are uses for the snorkel while on the surface, such as swimming back to the boat if you surfaced a good distance away. The best thing to do is dive the way you where trained until your experience is such that trying new techniques may be advantageous.
Snorkels attach directly to your mask with either the standard two-ring snorkel keeper or the newer clip device. Many of the newer and more expensive snorkels have the clip device.
Some snorkels have drainage valves that drain some of the water while surfacing, which mean there is less water to blow out. Some snorkels also have a cap that keeps water from entering the tube, meaning there is no water to blow out.
Look for a snorkel that has a comfortable mouthpiece, is oriented so that it is easy to use and breathe from, and that you can clear easy.
Cost: $10 to $100