Leasing is usually reserved for new cars only, and the best deals apply to merchandise that the manufacturer of the car is highly motivated to sell, since they have built too many of them .
Leasing works by predicting the depreciation of the car and having the buyer pay down only this depreciation, rather than the entire price of the car. When used car values dropped off a couple years ago, most banks got out of leasing, since they were losing big bucks when the cars came back to them.
Leasing also requires strong positive credit, which you will not have on your 18th birthday. In fact, you will need to bring along a qualified co-signer to get ANY car loan. Unfortunately, unless you have an amazing income and a really strong co-signer, you will get only a modest loan on a modest car, maybe half what you are looking to spend.
I am sorry to report that the newscasters are correct, as easy credit is now history.
Hopefully, you have a parent or close relative willing (and qualified) to co-sign for you, and hopefully you have good enough an income to budget for a car payment.
Don't hold your breath on the Mustang, though. Not to mention, 20K is out of line for a GT about to have its fourth birthday (since the 2010 Mustang
is out, the 2006 is actually four years old).