Hey there,
because I had to search a long time to collect all information, I made a detailed tutorial about how to connect a Raspberry Pi to a RGB LED-Strip or a WS2812 Strip.
It's available in English and German and should work on all current Raspberry Pi boards.
I hope you like it. I'm glad about every feedback!
Here you can find it: http://dordnung.de/raspberrypi-ledstrip/