If your looking for an American muscle car to tune, but are inexperienced and dont want to get in over your head, I have the perfect car for you to play with.
The Chevy Camaro Z-28 race car '69.
I'll tell you why.
its powerful, but not a nuclear weapon. And due to the fact its a race car, certain things to stabilize the framework come stock. If you drive this car stock, its already not that bad. Add some power parts, nice exhaust, sports computer, ect. Follow Motor City's guide as mentioned above, and go 1 change at a time, testing and retesting as you go.
the major benefit to this car, is that its capable of winning anything it qualifys for. It looks awesome, and you feel like a bad ass driving it. Why is that important? Because when your learning, when you see AND feel the rewards as you go, it keeps you at it, and keeps your attitude positive.
Keep your head up, master that car, and then you will feel better when you dive into other American beasts later.
good luck man.