Five-Step-Solution for those who find it intimidating to fill a 100-page sketchbook:
Step 1-Buy a cheap spiral bound, all-media sketchbook. Stay away from fancy books such as moleskins or leather bound books. If the sketchbook is luxurious or expensive, you'll be even more reluctant to draw in it and—god forbid— ruin its pristine pages.
Leave the moleskins to professionals (and to the trendy fools)