Well, if you're not using a scanner or a tablet, that leaves you with a mouse (or touchpad). I've seen some pretty amazing art done with mouse before,
such as these, but it takes some getting used to. You just have to practice everyday, drawing with a mouse, just like learning how to pick up a pencil for the first time and write with it.
I recommend getting a scanner though. You can get one for as cheap as $20, and you'll be able to share your work with the internet or digitally touch-up your artwork.