The amp rating on a power supply is a maximum value it can deliver, not a fixed value like the voltage. It can just be too low, never to high (to high may only make it unsafe to make mistakes, that is why you have to be careful when handling car batteries).
This is maybe the most basic thing in electronics there is. I suggest you put some effort in learning what voltage, current, power .... is, how they are related (Ohm's law) and how they behave in a circuit (Kirchhoff's laws), else you will not have too much fun with Arduino. All of these are described very well in Wikipedia.