Seu texto está muito bom. Algumas sugestões de melhorias:
Hello everyone, my name is Seth and I'm 19. I was born and raised in Portugal. I currently live and study in Brazil. After finishing high school, I entered medical school. I like going out with my friends, going to the beach and playing sports, especially surfing and tennis. I'm trying to learn English because I'll need it for work.
"How would the text of the American knack?"
Esta frase não faz muito sentido. Você poderia explicar o significado desejado?
"I also wanted to know if u have any errors in the text."
Uma melhor estruturação desta frase poderia ser: I also wanted to know (ou "I would also like to know") if there are any errors in the text.