If you're going to host an adventure path, you need only as much of the setting as the adventure path involves itself with since the adventure is, well, on that path. Anything off that path is outside of your prep. Players who play an adventure path should be aware that they are not, in fact, playing in a sandbox game. They are playing in a plot-based adventure that makes certain presumptions about character action (after adventure A, they will choose to do adventure B, and so on). This should be made clear at the outset so you can get their buy-in on that. You really don't want to be playing different games in the same group. If you decide to do your own thing that isn't plot-based and doesn't presume player choice or character action, you can go a couple of routes: You can create a setting or use a published setting. Creating your own setting can be a lot of prep or it can be very little, depending on how much improvisation and collaboration you want to do. (These are core GMing skills, so I advise you start practicing them now.) Published settings require only reading. "Setting" includes the world itself (geography, points of interest, etc.) and important groups, factions, and individuals in the world that are pursuing their own goals, motivations, and instincts. (I recommend Dungeon World's Front design for keeping track of this.) Those NPCs will attain their nefarious goals unless prevented in some fashion, usually by the PCs, if you view the PCs as the protagonists in the story you are collectively creating through actual play. Show the NPCs pursuing their goals by way of dramatic scenes the PCs can react to, then respond to their choices in fun ways that drive further adventure. Keep things moving and focus on what is interesting and fits the genre (heroic fantasy action/adventure); skip over the rest, advancing time as needed. It's very easy to let a sandbox turn into a "quicksand box," where it's slow and sluggish with boring focus on the mundane and nothing ever seems to happen. Portray a fantastic world, fill the characters' lives with adventure, and play to find out what happens - that's the name of the game. You should note that you only ever need as much of the setting prepared as the characters will experience e.g. if the campaign only takes place in the Valley of Spires, then you don't really need to detail every bit of the world outside of it. You just need the agreement of the players to play within that particular sandbox. If you feel comfortable with improvisation and collaboration, you can create what you need as you play with very little prep at all.
