Credentials

Say nothing.

If your book is about a student studying A-level English and it's a personal angle you can work in, then go for it.

Otherwise, say nothing.

I'm in the same boat as you (no writing creds). Say nothing. It's better than trying to fill in the blank with something that isn't quite relevant.

My book takes place in my hometown, so I wrote that down. I'm also a freelance writer, so I mentioned that too. But other than that, my bio is very very short. Don't worry about it too much. Just write a killer query.

If your story has been accepted then don't be ashamed to say "this is my first published story".

As an editor I don't want to see anyone's bio until I accept the story (it's in our guidelines but at least half the submitters ignore it) so in that instance there isn't a problem. But another magazine editor might want the bio up front (check the guidelines). I prefer a simple: 'Please consider my story "Blah Blah" for publication in Blah Blah,'.
'Thank you for considering . . .' or 'I'd like you to consider . . .', on the other hand, can be annoying.