Got your new arduino and want to start making cool projects?
Let's take a look at all the different ways yo can power it up in.
The Arduino microcontroller need 5V power supply to run, but the input voltage has to be greater than that so that any losses won't drive the voltage below 5V.
1. Power up using USB cable
This is the easiest way to power up your Arduino. Just plug in the USB to a computer (make sure that the computer recognizes the USB device, otherwise you will have less current) and connect the type B end of the USB cable to your Arduino board.
2. Power up using the barrel connector.
You need an AC-DC power adapter with an output between 6-12V. Plug it in and the Arduino jumps to life.
3. Power up using batteries
There are two ways to do this.
i) Use the Vin pin
Take a standard 9V battery, plug the positive terminal into Vin and the negative terminal into GND.
ii) Use the 5V pin
This is generally not advised because once you decide to use the 5V pin as input, you are also bypassing all the power safety steps of the ARduino that ensures that your board doesn't blow up. So you need CONSTANT 5V power supply for this. If you can achieve that using a battery and a voltage regulator then just plug in the positive terminal of your power source to the 5V pin, the negative terminal to GND and you are set.
(Source - Four ways to power up your Arduino Uno)