First and foremost, do not try and plan the whole adventure in one sitting unless you want to drive yourself insane. Second, try thinking of the geography, and maybe topography. Where are your characters? What cities are nearby? What threats lurk close? Third, start making dungeons, always start with something simple. Our groups first dungeon was a bugbear with some goblins in a cave holding our friends for ransom and saying he’d kill them, his only goal was hoping people would pay to get them back. Fourth, familiarize yourself with the world you made. What would residents of your village request of you? Finally, think of an event worthy of a small adventure, and make sure you come up with a good villain with a decent motivation.