1. Prepare your Raspberry Pi: download the last image of Raspbian from http://www.raspberrypi.org/downloads, unzip the archive and flash it on a sdcard with dd
dd bs=1M if=/path/to/2012-12-16-wheezy-raspbian.img of=/dev/sdX
2. Put your sdcard in your Raspberry Pi, connect it to your TV and your LAN, then boot it. You need to use a graphical interface, so run:
3. Open your terminal and install PyKaraoke, running:
sudo apt-get install pykaraoke
4. Prepare your karaoke machine: copy all your karaoke files in a new folder into Raspberry Pi, e.g. ~/karaoke, and import this folder in PyKaraoke. Open PyKaraoke and click on Add Songs, this window will pop up:
5. Test your machine: try searching for your songs using PyKaraoke, with a double click on one of them should pop up lyrics on your TV.
Well done, now you're ready to rock :)
Thank you Raspberry Pi!