user34368: I want to control (only on/off) a small water pump (5 V, 200 mA max.) with my Arduino. 
I read something about NPN transistor, because the output pins of the Arduino can't supply enough current to the pump. However, I don't know how to choose the right transistor and how to connect it with my Arduino and my water pump.

user33045: To use an NPN, connect a power rail of 5V to the pump's positive terminal, and from its negative terminal wire it to the transistor's collector pin. Wire emitter to GND, and place a >=220 ohms resistor between your arduino control pin and the transistor base. 
If instead using an N-channel MOSFET, wire load to drain, source to GND, and use a 4K7 pull-down on gate. 
Also add a diode, with its anode at the pump's negative terminal, and cathode (white ring) at the positive terminal.

34368: <p>I want to control (only on/off) a small water pump (5&nbsp;V, 200&nbsp;mA max.) with my Arduino. </p>

<p>I read something about NPN transistor, because the output pins of the Arduino can't supply enough current to the pump. However, I don't know how to choose the right transistor and how to connect it with my Arduino and my water pump.</p>
 33045: <p>To use an NPN, connect a power rail of 5V to the pump's positive terminal, and from its negative terminal wire it to the transistor's collector pin. Wire emitter to GND, and place a >=220 ohms resistor between your arduino control pin and the transistor <em>base</em>. </p>

<p>If instead using an N-channel MOSFET, wire load to drain, source to GND, and use a 4K7 pull-down on gate. </p>

<p>Also add a diode, with its anode at the pump's negative terminal, and cathode (white ring) at the positive terminal.</p>
