They found a polyp in my stomach today. Should I be worried if it's cancerous?

Not a lot/remove it. Gastric cancer usually arises from an ulcer that doesn't heal well or at all, and often there is an infection with h pylori, a known factor in the development of gastric cancer. Polyps may be hyperplastic, or precancerous, as with adenomatous polyps. They can lead to cancer so they should be removed, or at least biopsied. I would see your doctor to get this done if you didn't already.
Probably not. There are many types of polyps: hyperplasia, adenomatous, fundic gland type, etc. Only a small percentage are cancerous, and for someone your age, that is most unlikely.