Choose a sketch pad with a paper that works with the pencil that you'd like to use. Fragile paper rips if you use a hard lead, while rough paper produces lots of dust with soft lead.
Select a paper quality that matches the use you have in mind. Choose watercolor paper if you're going to use watercolors or a pad of newsprint if you're going to generate a lot of quick sketches. Ask the art store clerk for advice if testers aren't available.
Choose a graph paper pad if you're going to make measured sketches.
Select a bound book if you're going to make sketches and notes to record a trip and keep them as a memento. Choose one that stays open without damage to the spine.
Pick a sketchbook with replaceable, loose paper if you're tentative about your sketching talent.
Choose a sketchbook that doesn't intimidate you with a cover more fabulous than the sketches you'll make in it.