To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
wellywales: Keep playing and winning and you'll open up a gap. If the gap gets to three Steps you won't have anyone left to challenge, nor will anyone be able to challenge you until they've climbed up. This can be good or bad, lol. I view it as a breathing space.