There are lot of information about uskng rasberry pi. Here are steps that I had to go thro to setup rasberry PI as an xbmc center.
Head to Adafruit get the 512MB version of rasberry pi. Please get your pi from Adafruit. Mostly Pi is bit pricey in other websites, but I like Adafruit and shipment is super fast. I would recommend putting a order Tuesday and you might get your shipment Friday evening,
Secondly get your gadgets for the Pi.
Pi is powered by a USB or you can get a charger from adafruit. I ended buying a dingus from bestbuy that was a 4 port belkin USB HUB.
Here is a picture of how you will be able to power it.
Just to be clear the power connector for Pi is the one that you use for charging a BB or a Kindle. This is a very small charger. Make sure all the LED's on the Pi are lighting once you power it up. Now we know that Pi is powering up and running.
There are lot of awesome things that can be done with Pi, but I found the XBMC setup is the best for my needs. So in order to make the Pi bit wiser goto this link. You will get the latest version of XBMC linux/mac.
I ended buying a 8 GB SD card from best buy for $9 to install, Ensure you get the SD card that has a UHS-1 Class 10 memory. Your Pi sucks only 5 watt of power and there is no hard disk. This card is heart and soul of your Pi.
Insert a HDMI cable and an ethernet cable to the Pi, it is all set. Here is the final picture of the entire setup looks like
You have two options to control the Pi. Using a keyboard by connecting to the USB connection or an iOS app. I prefer controlling my eco-system using an iOS app; Here is the link to get to the app. Now you should be able to even Airplay your content using Rasberry Pi.
If you want to use your Pi for other reasons, simply switch the SD card, one device to rule them all. It is good to hack and play with an electronic board after college.