1. If you havent already, go to your BIOS, and turn on WakeOnLAN (it varies, look for it.) If your network card is onboard, your set for step 2, otherwise there is probably a cable that should go from your network card to your motherboard, though this is not always the case.
2. Back in ubuntu, kubuntu, xubuntu, w/e, we now need to make a script that will run every time the computer is started, because this command only lasts until the computer is turned on again once.
2a. Find out what network device you want to have the computer wake-able from, usually all, which is just one. If you have more network devices in your system, 9 chances out of 10, you already know what they are called.
You can NOT wake up a laptop or computer that is only connected via wireless with wake-on-lan, unless the bios has a method for this, this is very rare, and I do not garuntee this howto will work in such cases.