While some sports like diving need prior training, snorkelling is far simpler and doesn't mandate that. Just jumping right in with no idea what you're doing, however, isn't ideal, and it is best to pick up the basics beforehand. You can always practice with your equipment before you start, either in a swimming pool or in a calm and shallow part of the ocean. Practice breathing with the snorkel while you are in a controlled environment to avoid getting overwhelmed by it out at sea. This way you will also be able to learn how to perfectly fit your mask and where to place your snorkel, and how to stay calmly afloat on the water.