You've never done this, so:
1. Make a simple straightforward story with:
a) Lead-in (ie. "The Inn Scene", where your players get their goal.)
b) Happenings (What happens when your players are midway there. Two at max since you're just starting out.)
c) Point (ie. "The Boss Fight", even if there are no Boss or Fight. The highlight of your scenario.)
b) Epilogue (ie. "The Happily Ever After", or loot splits.)
Yeah, I am telling you to make it one-path and boring. Cause you're just starting out, and it is best that you get used to the hang of it before you add variations, subpaths and secrets.
(Plus, if it comes out "boring" or not, really depends on how the GM runs it more than how the scenario is set up.)
(Plus plus, there's a big chance that your player might either just wander off and do something else besides the goal, or they just miss all your well-crafted secrets and hints.)
Don't go big, go comfortably manageable. You can always go big later.