Aye, with the pen tool it's -really- hard to draw something without a sketch already underneath.
The way I often do it is to draw my picture and then scan it into my computer, put it in Photoshop and then zoom in a bit and use the pen tool on a layer over it.
With the tablet it's the same but without scanning, just sketch straight in Photoshop and then either use the pen tool or zoom in more and do the lines with the tablet.