From: https://learn.adafruit.com/adafruit-16x2-character-lcd-plus-keypad-for-raspberry-pi/usage

NOTE: if i2c does not appear to work,
edit /boot/config.txt and add: dtparam=i2c1=on
(cf. http://raspberrypi.stackexchange.com/questions/27073/firmware-3-18-x-breaks-i2c-spi-audio-lirc-1-wire-e-g-dev-i2c-1-no-such-f)

* edit /etc/modules, add i2c-bcm2708 and i2c-dev
* sudo apt-get install i2c-tools python-smbus
* confirm that the lcd was detected by running:
    sudo i2cdetect -y 0 (if you are using a version 1 Raspberry Pi)
    sudo i2cdetect -y 1 (if you are using a version 2 Raspberry
  you should see it show up at 0x20
* sudo apt-get update
* sudo apt-get install build-essential python-dev python-smbus python-pip git
* sudo pip install RPi.GPIO
* git clone https://github.com/adafruit/Adafruit_Python_CharLCD.git
* cd Adafruit_Python_CharLCD
* sudo python setup.py install
* test by running:
    cd examples
    sudo python char_lcd_plate.py

Look at the contents of the char_lcd_plate.py file to see the basic usage of the character LCD plate class. The basic usage is to import the library and create an instance of the Adafruit_CharLCDPlate class. The char LCD plate class is smart enough to know how to talk to the character LCD plate without any configuration or parameters. 

Once you have an instance of the Adafruit_CharLCDPlate class there are a few functions you can call to interact with the display:
message(text)
Print the provided string message to the display. The text string can include linebreak characters ('\n') and will move to the next line when found in the string.
clear()
Clear the display and reset the position of message printing to the first column and first line.
set_color(red, green, blue)
Set the color of the red, green, and blue backlight LEDs. Each color value should be a 1 for on or 0 for off. For example to set a red backlight call set_color(1,0,0) or to set a purple color call set_color(1,0,1).
You can find a more detailed write-up of the library in this character LCD guide (see the Usage and Raspberry Pi Char LCD Plate pages).

more at: https://learn.adafruit.com/character-lcd-with-raspberry-pi-or-beaglebone-black/overview
