I am 19 and the last time i ate fish i was probably 3 years old. can i try sushi or would i get a reaction cause i'm not used to seafood?

Unlikely: Unless you have a personal history of food allergies to fish or strong family history there should be no reason to be concerned about having a reaction. While a common food allergy is to shellfish (shrimp, lobster, crab) it is less common to have allergies to fish. You should be fine but certainly be aware of any reactions such as lip or tongue swelling, rash, itching or shortness of breath.

Sushi: unless you have a well-known allergic reaction to fish, you should be safe to eat sushi or any other fish