Read the Cubase manual if you've never used a sequencer before. You'll probably have to at some point and it will make your life easier. Also, if you don't have one and have the spare cash and are used to working hands on with instruments, consider picking up a cheap midi keyboard controller. Personally, I find it a lot harder to do melodies with a mouse than I do with a midi keyboard in front of me. Keeps me from getting bogged down in one function and losing flow.
If you don't have the Cubase manual, btw, I believe it is floating around on the net in PDF format.