Well, it doesn't matter where you are going to put it. If I were you, I would use it like this. "안녕하세요, 전 멕시코계 미국인인 레이첼이라고해요" - formal "안녕, 난 멕시코계 미국인인 레이첼이라고해" - informal

안녕하세요 레이첼입니다. <-- This is a formal one and usually your introduction should be formal. More information is recommended. 레이첼 <-- Only using your name. you cannot introduce yourself effectively. 안녕? 레이첼이야 <-- This is informal and it is not good for introduction. If you want to use informal style, you should write more about yourself. Why you are studying Korean. The motivation of learning Korean. Experiences about Korea or Korean people, Korean culture, or Korean Food. etc.