If you want detailed answer, post your plasma model (on back sticker usually) or post a picture of the inputs/outputs.
You could use composite video (you will need a composite to bnc for the plasmas bnc connector) which should be yellow on your xbox component lead (yellow red white into scart plug).
You could use VGA (
http://xbox2vga.notlong.com).
You might be able to use dvi, search for adapters if you want it.
Look for any audio conectors on plasma, it may have pre-amp, if so you dont need another amp for sound, just speakers via speaker wire from plasma.
If not, you just need to get the phonos (red & white) off to your amp, I would get a phono joiner and another long enough phono lead and plug my xbox phonos into the joiner then lead and use that.
If you cant do any of these, let me know why and what you have and ill work it out further.
As always, avoid maplins and pc world/currys if you dont have money to burn they will rob you in daylight for cables/adapters, buy from online cheapos or local small electronic dealers.. i used to pay 50p for a phono joiner, i think maplin charge 1.99 or something.