The PiTFT requires kernel support and a couple other things to make it a nice stand-alone display. We have a detailed step-by-step setup for hackers who want to tweak, customize or understand the PiTFT setup. If you just want to get going, check out the following for easy-install instructions!
If you want to start with a fresh image, we have one for Raspbian - click here to download it and install into a new SD card. Unzip and follow the classic SD card burning tutorials
For further information on calibration and installation, please visit the Adafruit 3.5" PiTFT guide.