Your monthly payment and down payment are going to depend on your credit score. Being 18 years old, you don't have a lot of credit built up at this point in time so you'll probably need to have a cosigner. It also depends on how much the car costs. If you're getting a base model with no extras you're going to be paying significantly less than a GT with all kinds of premium options. Your best bet is to visit a dealership, see what they have that's in your budget, calculate the monthly payment and find out where you stand credit wise. Don't get talked into anything that you don't want, car salespeople are good for that.
As far as insurance, it's most likely going to skyrocket for several reasons: 1) You're under 18, 2) It's a Mustang, and 3) You'll be paying a loan which will require you to carry full coverage insurance. Shop around for some insurance quotes, as it's going to depend on many factors such as your driving record, where you live, etc.
Now I just wanna give you some advice based on my own experiences, it's completely up to you whether or not you wanna take it. When I was 18 I got a brand new car at the insistence of my father. Yeah, it was nice having something brand new, knowing it was dependable and warrantied. But you know what else? Every dime I made went to either the car payment or car insurance and for four years I had barely any money to spend on anything else (and I worked two jobs with decent pay). Really think this through before you make this kind of commitment. Is this job (and the pay scale) pretty much guaranteed for the life of the loan? Cause if you suddenly lose your job or get a pay cut, it's gonna make it very difficult to afford your payment and insurance.
That said, I can agree with the Corvette owner who says buy what you love. I recently bought a 2002 Mustang
that I am absolutely in love with. The best part? No payment.
Just make sure you think everything through and find out exactly what insurance and your monthly payment will cost before making any commitments, you don't want to end up hating the car because your life revolves around working to pay for it. Good luck.