Is this a cracker or an original? IF it's the original, the only way to gfet your head around it is to go through the manual. Start with the quick setup booklet then graduate onto the full manual. If it's a cracker, then download the pdf manual from steinberg.com (once you get good at it you will probably want to get the original anyway!)
I found a couple of great books on amazon that show you how to get started right through to mastering for a cd. Get a few of these, they are the only quick way to learn easily. Best buy in my collection: "Fast Guide To Cubase SX" by Simon MIllward (make sure you get the SX3 version)
It is far too difficult to explain in a forum and will take you a while to get up and running even with the books. Just stick with it and you will be making music after about a month (you will find that you may have to download some VST plugins and/or invest in some patches). Also, you will have to get a midi keyboard that can connect to you pc, you also need a proper soundcard and your pc must have enough RAM to be able to handle the software.