Where to start is the big question, and Cubase is a big package. First of all, make sure you're soundcard is setup correctly for audio and midi - see 'Devices'. Once you've done that, save youself a whole lot of hassle and create a template. Audio in (from your guitar) should be set up on a mono channel (unless you are recording with stereo effects - not advisable anyway). You aso want to set up a midi channel for drums. Once you get a project template sorted, save it!
Another major tip is to create a folder for each song you do - if you don't you will find all hell breaks loose after a few weeks of recording. Also, when you recod something and delete it, it's not really deleted at all... make sure you regularly open the Pool window and properly delete the deleted items, if you don't you will find your pc running out of memory pretty quickly... This is already sounding to complicated... Best advice of all is to buy one of those Cubase tips books... I got one on amazon and it never leaves the room (a very worthwhil investment).
BTW, if you are using a cracker copy, don't bother asking questions in any of the Cubase forums, you will get flamed to hell and back again!