The Raspberry Pi has all kinds of setting hidden away inside its configuration file. If you don't want to mess with that, PiCon is a program that allows you to easily set up your Pi without mucking around in the config.txt file.
PiCon is cross-platform and it simply creates a config.txt file that you can save to your Raspberry Pi. It's packed with all kinds of options for the Raspberry Pi, including forcing HDMI, changing aspect ratio and rotating the screen. You can do all this stuff from the command line on the Pi if you want, but PiCon makes it easy to see all your options in one place. After you create the file, just upload it to your Raspberry Pi, and you're all set.