Turn everything on and you should find that when the Pi boots, it will take you through a few configuration steps including selecting your language.

Setting up WiFi

If your Raspberry Pi is connected to the Internet using a cable, then it should automatically connect itself to your network using DHCP and find itself an Internet connection.

If however, you want to use a USB WiFi adaptor, then you need to configure this, by using the Raspbmc Setting program, that you will find in the Programs section of XBMC.

Select the WiFi option and scroll down to enter your SSID (WiFi network name) and WiFi password.