Ever since the dawn of Arduino, one question has been asked over and over again: "what kind of DC adapter can I use to power my Arduino?"
The short answer is: 9 to 12V DC, 250mA or more, 2.1mm plug, centre pin positive.
The long answer is that an off-the shelf Arduino adapter:
These important details are often contained right on the adapter. Here's a picture of an adapter ideally suited to powering the Arduino. The important info is underlined in red here:
As you can see, it has all the right stuff: 12V, DC, and a little picture that shows you that the middle pin is positive.
Current rating: Since you'll probably be connecting other things to the Arduino (LEDs, LCDs, servos) you should get an adapter that can supply at least 500mA, or even 1000 mA (1 ampère). That way you can be sure you have enough juice to make each component of the circuit function reliably.
One final note. If you check the "Tech Specs" tab on the product page for your Arduino board you may find the input voltage limit listed at as much as 20V (specs vary depending on which board you have). This is an absolute limit. Powering your board at either end of that range is NOT recommended. The reason you don't want to power your board at a voltage near the upper limit is because most of that voltage will be converted to heat. Our advice is to stick with a 9V or 12V DC adapter.