formalizing debt

[thenatureboy420]

What is the name of your state (only U.S. law)? Illinois.

So my girlfriend is living with me, and agreed to pay half of everything. She has been unable to find a job and has racked up a debt of a little more than $1000. I have kept very informal track of all debts incurred.

I was thinking about getting her to sign something indicating her debt to me. Things are going well, but I don't want to get in a big fight or something and have her refuse to pay. I expressed my concerns and she said she would agree to sign something. The amount of money is not a debatable issue, as we both agree on the amount.

My question is how do I format an agreement that, in the worst case scenario, could be used in court to make her pay? Is there any way this contract could change each month as her debt increases or decreases? I assume we both need to sign, but what else is necessary? A witness? At the very least, I'd like to formulate a legitimate enough document that could coerce her to pay if she tried to refuse, something she would think would work in court.

What would the creation of the type of document I am describing entail?