text: Q:
  
  What NPN transistor should I use?
  
  I need to control a 12 V 5 A (or 3 A if need be) with an Arduino UNO which has 5 V and 40 mA. Unfortunately I don't know how to read transistor data sheets. What NPN transistor should I get for this project?
  
  A:
  
  You shouldn't use an NPN transistor for this project. There are parts out there that will work by an N channel, logic level MOSFET is a better choice. It's \$R_{DS}\$ in saturation will be very small, and it won't waste power as heat. Take a look at a part like this one. \$R_{DS}<10m\Omega, P=I^2R = (5A)(10m\Omega) = 50mW\$ That is pretty good.
  You'll implement it like this:
  
  simulate this circuit – Schematic created using CircuitLab
  
  