How can I stay to the USA?

Since you are 15 and not an adult yet, your best bet is to do a student exchange program, when student from different countries come to study to the US (I'm sure your still in school). Then you will get experience in living in the US, and also you will improve your English, and you will have a place to stay (probably with a family that is willing to take care of exchange students, but Im not totally sure how it works). Then you can take it from there. Finish school in the US or come here when you graduate in Greece. Then you can apply for a student VISA and go to chef school here in the US. Later you can apply for a green card. United States in definitely nice and there are a lot of places you can live at, from really big cities like New York to tropical gateway spots like Florida and California. I wish you luck! My dad had a dream just like yours when he was your age, and now we all live in the States. Good Luck!