키난, 안녕하세요~
You wrote well, but you switched between informal and formal in the same sentences.
Formal would look like this:
안녕하세요. 저는 키난이에요. 저는 열일곱 살입니다. 만나서 반갑습니다.
Informal would look like this:
안녕. 나는 키난이야. 나는 열일곱 살이야. 만나구반가워.
It would be fine for you to use 반가워요 when speaking/writing formally, but 반갑습니다 is more polite, so I used that instead. Feel free to ask me any questions if this didn't make sense.
감사합니다~
1. 나의 소개
안녕하세요.
나는 키난입니다.
열일곱 살이구요.
만나서 반가워요.
Hello.
I'm Keanan.
I'm 17 years old.
Nice to meet you.