Too many variables. Go to kbb.com and see what they retail for in your area. determine the amount you can put down and then figure that since you are 18, and haven't much credit history, that you will have a low FICO score (credit rating) even with a co-signer. That means a higher interest rate - perhaps 9% or more. There are lots of on-line payment calculators.
Do not - repeat, DO NOT - max yourself out on the purchase price. You will have insurance issues - over $100 a month, maybe more if you get a bank loan and they require full coverage. Then there is gas and maintenance to consider. So, make a budget to see what you can afford. You may want to think of an older model for now so you can save up a bit towards a nice truck later. The Tundra gets only fair mileage so make a generous gas budget unless you have friends who readily are known to chip in for gas. Good luck.