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.