Car interest rate help?

ok well im 18 and just got approved for a credit card so i have no established credit i need a car and i really want an 08 civic si coupe (MSRP 21500)...my question is what would be my interest rate if i put 8000 down and get a co signer with below average credit...my so signers credit is bad but not horrible...so what do you guys think my interest rate would be and estimated monthly payment.