Don't swim your first Triathlon without your goggles! It makes sense to wear your goggles when you train in a pool to protect your eyes from the chlorine in the water. When you do your open-water training, you'll want to wear your goggles again to protect them from the sea-water. If your swim event will be in a lake, you may not need them. Some swimmers find that wearing goggles improves their visibility under the water, which helps you from getting disoriented.
You're not limited to wearing goggles. Many triathletes wear swim masks when they're training and racing. They feel that the masks afford them even better visibility than goggles would. If you normally wear glasses or contacts, look into getting your goggles with vision corrected lenses.
While fitting goggles isn't as hard as fitting masks, you'll still need to take a fit and feel approach rather than buying your first pair online. The goggles can be adjusted around your nose bridge but you may find that some brands fit you better than others.
