To get started we first need to install Raspbian on to the SD card. Instead of reinventing the wheel and writing a complete guide on how to do this, the Raspberry Pi Foundation has created an excellent Getting Started with Raspberry Pi guide in PDF format. I feel that this is by far the best "Getting Started" guide on the internet and would recommend that you use it.
Once you have the latest version of Raspbian installed and your Pi is up and running, we need to install Raspbmc onto the SD card you installed Raspbian on. For Windows users, this process is quite simple and just involves you downloading and installing the Raspbmc Installer. Simply download it to your desktop and with your SD card plugged into your PC, run the installer. If you are running Linux, the process is a little more complicated, and you will need to run some commands from the terminal. Mac users are able to take advantage of an installer similar to the one used for Windows.
For Linux users in the terminal type:
wget http://svn.stmlabs.com/svn/raspbmc/testing/installers/python/install.py chmod +x install.py
Once that command has been ran, you will need to run the installation script using root privileges. In the terminal type:
sudo python install.py
For Mac users I was unable to test this program out as I do not own a Mac, but after a few searches, I feel comfortable suggesting it. Simply head over to XBMC Hub and download their Raspbian Mac installer.
Now that you have Rasbian installed on your SD card, you can remove it from your PC and plug it back into the Raspberry Pi and proceed on to the next where we will connect the Raspberry Pi to your TV and begin the XBMC download process.