You can use the output from the Arduino 5v pin to provide a limited amount of power for a breadboard - say 200 - 300 mA.
If you need more power than that you will need a separate power supply. The ground from the separate power supply and the Arduino ground should be connected.
The digital I/O pins on the Arduino can only supply a max of 40mA and 20mA is more realistic. And I think the max for all digital pins must be less than 200mA.
If you want to use one of the digital pins to light an LED remember to include a current limiting resistor - anything from 470 ohms to 4700 ohms should be fine.
...R