How do I step down voltage and current in a DC application?
Basically there are two types of voltage regulators: linear and switching. Linears are bad: if your 5 V needs 2 A it will also draw 2 A from the 11 V input, and the difference, 6 V x 2 A = 12 W, will go up in heat.  

You need a switcher. Since you're going from a higher to a lower voltage that will be a buck converter. (Lower to higher voltage is called boost.) Buck converters can reach efficiencies higher than 90 %, then the power loss will be less than 1 W. Here is a selection of switchers that fit your requirements.  

For maximum efficiency I would suggest that you remove the NPC1117 voltage regulator from the Arduino (I'm referring to the Uno, other Arduinos will have a similar linear regulator), and connect the output of the switcher to the 5 V net directly. Otherwise you would have to supply at least 6 V, and lose 800 mW in the NPC1117 regulator (it can supply maximum 800 mA).

Whenever possible try to run as much as possible at lower voltages, like 3.3 V. A switcher going from 11 V to 3.3 V will have a lower efficiency than one going to 5 V, but you'll gain by less dissipation in the 3.3 V devices. I even think (not checked!) that Arduino can run at 3.3 V.