In my story when should I introduce this about my protagonist?

[Cazz *McFly Pioneer*]

Yes I know that you shouldn't start your NaNoWriMo story before Monday, but I've decided to write the first chapter (that's all I promise).

Okay so my story is about this 17 year old girl who sold her soul to a demon when she was 13 years old. Her mum was gravely ill and the demon (she didn't know he was a demon at the time) promised her that her mum will become well again, if she promises in five years time, on her 18th birthday she will give him something. She agrees and he gives her a necklace with a key shape charm attached to it.

She then learns about the deal she made but she doesn't yet know that demons store people's contracts somewhere secret and it there is a key which unlocks the contract.

After the deal was made she started to see evil spirits and demons and was sent away to a religious boarding school where nothing evil could get in.

During the story there are bodies of girls who are killed. Every one of them look the same. They are all the same age and have long blonde hair and blue eyes, they also have dark secrets but the protagonist doesn't find that out until later on in the story.

Not this is my question...
The beginning of the story starts with the protagonist wakes up to find the police at the school. she looks out her window and sees a dead body of a girl, the fourth victim. Her best friend wakes up and they talk about the murders.
The next scene in the same chapter goes to class the next day where the teachers explain about the murders.

Should I change the first chapter and explain to the readers about the demons and the protagonist's contract, or could I introduce that in the second chapter?
I know I said 'nothing evil could get in' then I said that people were being killed.
Nothing like that happened until the protagonist started the school and that is when evil *could* get in.
Oh my god! The title of this question doesn't make sense...
I paid so much attention to the details that I forgot about correcting the title..
Oops lol

 

[Julia]

Go half and half. Reveal some of the details early on, and reveal more and more as the story goes on, until the whole picture is visible. It would give the story continuing suspense--tension about the murders, and tension about what exactly is going on that we haven't been told yet.