Because the anime uses flashback techniques.
The reason Minato appears in the first episode of Shippuden is because: the anime uses flashback techniques. Minato is a character in Shippuden.
Namikaze Minato is the fourth generation Hokage of Konoha Hidden Village in the Country of Fire, a disciple of Jiraiya, one of the Konoha Sannin, and the father of the protagonist Uzumaki Naruto. He was so powerful that he developed the Rasengan and died in order to protect the village and use the ghoul to seal the Nine-Tails.