Welcome, @Adz123 to the myFICO forums!
The first question is, does the car dealer carry the loan or are you going through a financial institution? If you are going to purchase a car from a "we tote the note" or "buy here, pay here" car lot, you will most likely not have the loan reported to the three credit bureaus to build credit.
Second question, over what period of time are you going pay back the loan and at what interest rate? This information, along with how much is being financed is needed to calculate your monthly payment.
A loan of $10,000 financed at 20% over 72 months would give you payments of $239.53 a month, costing you $17,246.16 over the 72 months.
A loan of $9,000 financed at 20% over 72 months would give you payments of $215.58 a month, costing you $15,521.76 over the 72 months.
If I understand you correctly, the dealership is going to find you financing?
A few thoughts:
First, negotiate the out the door price of the car before you look into financing. Never talk about what you can afford to pay a month.
Second, most loans I know of that are 72 months are for cars that are $35,000 or better. I don't think you are going to get financing for $9,000 from a sub-prime lender the dealership is going to find for you for 72 months.
Finally, as your FICO scores improve as you pay down your loan, you should think about refinancing your loan for a better interest rate.
Good luck in buying your car.