Determining what your monthly payment would be is fairly simple. A general rule of thumb is with good credit (700+) your monthly payment will be $20 for every thousand financed. Less than perfect credit (<600) winds you up at around $30 per thousand.
Less than Perfect Credit
-Note every $1,000 you put down will reduce your monthly payment by about $20.*
-Also every $1,000 negative equity you have in your trade will increase your monthly payment by about $20*
Leasing is different than financing in that you are not paying for the full price of the car. Your payments are ONLY off of the difference between the selling price of the car and the determined residual value it will be when you turn it in. So if the car you are leasing is $20,000 and the residual value determined is $15,000 after 24 months, you are only making payments on the $5,000 difference. General rule of thumb for leasing is $30 per thousand leased.