This text is based on the Getting Started text from the official Arduino site under a Creative Commons Attribution-ShareAlike 3.0 License.
Get the latest version from the download page (http://arduino.cc/en/Main/Software).
When the download finishes, unzip the downloaded file. Make sure to preserve the folder structure. Double-click the folder to open it. There should be a few files and sub-folders inside.
The Arduino Uno automatically draws power from either the USB connection to the computer or an external power supply. If you're using an Arduino Diecimila, you'll need to make sure that the board is configured to draw power from the USB connection. The power source is selected with a jumper, a small piece of plastic that fits onto two of the three pins between the USB and power jacks. Check that it's on the two pins closest to the USB port.
Connect the Arduino board to your computer using the USB cable. The green power LED (labeled PWR) should light.
Installing drivers for the Arduino Uno with Windows8, Windows7, Vista, or XP:
When you connect the board, Windows should initiate the driver installation process (if you haven't used the computer with an Arduino board before).
On Windows 8, 7 or Vista, the driver should be automatically downloaded and installed. You can check that the drivers have been installed by opening the Windows Device Manager (in the Hardware tab of System control panel). Look for a "USB Serial Port" in the Ports section; that's the Arduino board.
Double-click the Arduino application. (Note: if the Arduino software loads in the wrong language, you can change it in the preferences dialog. See the environment page (http://arduino.cc/en/Guide/Environment#languages) for details.)
Get the latest version from the download page (http://arduino.cc/en/Main/Software). When the download is finished, double click the .zip file. This will expand the Arduino application.
Copy the Arduino application into the Applications folder (or elsewhere on your computer). No drivers are required to be installed.
The Arduino Uno automatically draws power from either the USB connection to the computer or an external power supply. Connect the Arduino board to your computer using the USB cable. The green power LED (labeled PWR) should go on.
A dialog box will appear telling you that a new network interface has been detected. Click "Network Preferences...", and when it opens, simply click "Apply". The Uno or Mega 2560 will show up as "Not Configured", but it's working properly. Quit System Preferences.
Double-click the Arduino application. Note: if the Arduino software loads in the wrong language, you can change it in the preferences dialog. See the environment page for details (http://arduino.cc/en/Guide/Environment#languages)
Getting Started on Linux depends on your particular distribution. Details can be found here (http://playground.arduino.cc/Learning/Linux).
Open the LED blink example sketch: File > Examples > 1.Basics > Blink.
You'll need to select the entry in the Tools > Board menu that corresponds to your Arduino.
Selecting an Arduino Uno
Select the serial device of the Arduino board from the Tools > Serial Port menu.
Select the serial device of the Arduino board from the Tools | Serial Port menu. On Windows this is likely to be COM3 or higher (COM1 and COM2 are usually reserved for hardware serial ports). To find out, you can disconnect your Arduino board and re-open the menu; the entry that disappears should be the Arduino board. Reconnect the board and select that serial port.
On the Mac, this should be something with /dev/tty.usbmodem (for the Uno).
Now, simply click the "Upload" button in the environment. Wait a few seconds - you should see the RX and TX LEDs on the board flashing. If the upload is successful, the message "Done uploading." will appear in the status bar.
A few seconds after the upload finishes, you should see the pin 13 (L) LED on the board start to blink. Congratulations! You've got Arduino up-and-running.
If you have problems, please see the troubleshooting suggestions http://arduino.cc/en/Guide/Troubleshooting).
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