Buy a raspberry Pi model b:
buy an hdmi cable:
buy a network cable (you probably have one in your BT modem box in the attic:
You might have one, if not buy the power supply:
Buy an SD memory card that works with Raspberry pi:
List of working SDs - http://elinux.org/RPi_SD_cards
This one seems to be the cheapest it's on sale!! - (SDSDU-016G-U46) - http://www.memorybits.co.uk/shop/sdcard/sandisk-16gb-ultra-(30-mbs)-sdhc-class-10-sdsdu016gu46/22933
Wait for postman... dum dee dum...
Stick the SD card in your PC / laptop, follow the following very simple instructions:
Stick SD card in Raspi, plug in ethernet, plug in hdmi to raspi and tv, plug in power. Watch as your TV turns into something amazing.
Spend the next few weeks reading how to configure XBMC and watching geeks on youtube ;)