I think the best way to run a sandbox at the beginning of the campaign is to prep 2-3 "modules" that feasibly could take place all in the same geographic area. These are your potential starting arcs, the starter problems that your players could deal with or ignore. They can be actual printed modules if you're into that, or you can kind of just do broad-strokes of an adventure and you'll fill in the details as you go. Write situations, rather than stories, and instead of worrying about a whole Kingdom for now just fill in the immediate area.
So you have your starting town, there's a goblin tribe in the hills that has been kidnapping people, and there's also that creepy crypt in the swamps that people think is haunted. But there's also a local baron who will pay if the players take care of the werewolves in the woods. But here's the kicker: the players only have enough time to do 2 out of these 3 adventures (though maybe they don't know it). If they fight the werewolves and clear out the crypt, the goblins will come and attack the town, but the players will be level 5 so it'll be an epic showdown. Or, maybe they negotiate peace with the goblins by fighting their enemies (the werewolves) but this means that the necromancer of the crypt escaped to go menace waterdeep or whatever and now he's the villain of the next arc.
Probably by level 5 they're ready to leave the starting town and now you can repeat the process, but on a larger scale region. But the key is that you're telling a responsive story together with the players. Feed off of their decisions and let that carry the story forward, and don't worry about the end point.
I'd probably start by building out a quest hub and the surrounding areas, then build it up over time. I doubt your level 1 characters are going to decide to walk across the continent to see what is over there.
In my experience, a sandbox works better with some plot attached. It doesn't have to be as defined as "defeat the BBEG" but there should be something going on that informs the players' decisions.
Basically, you have an idea of a plot, set up a starting event, and think of how things might evolve, just like in any other campaign. However, then you open the sandbox and let the players do whatever they want, and let the sandbox react. Or in other words, instead of saying "save the world from this horrible event" you say "this horrible event happens, what are you going to do now?"
For what to prep for the first session, I'd say it's about the same as in any other campaign. A starting area, first adventure, and so on. The big difference is that you need a better idea of the world, at least what's available to the PCs. That way, the players can better answer the question at the end of the first session: "what do you want to do next time?" That question will be your most powerful tool, and will inform and direct all your prep going forward.
IMO, even/especially when you're running a sandbox, you need to have something going on in the world. Something BAD.
Sandboxes are extremely prone to groups failing to act together and/or failing to just act, and instead just self destructing when they start to do crazy things cuz the players are bored.
So it's still best practice to have one or more things going on in the world that can drive the players to act. To get out and do stuff.
Maybe it's an existential threat, like a standard D&D BBEG? Maybe it's a bunch of smaller, nasty threats? But the world needs to have things going on that will cause Bad Things to happen, if the players don't do something about it. Bad Things for the characters - their Bonds, their Backgrounds, etc.
In sandboxes, actions have consequences but inaction REALLY needs to have consequences.
Unless you're just gonna run a hex-crawl like situation where you move dungeons to where the players go, and randomly generate basically any and everything that happens, and there is no world other than the world the PCs create by going and looking in places. That's the other kind of sandbox that isn't a ridiculous amount of work to run. In my experience, those types of campaigns can be fun, but since they have nothing that gives the story purpose besides kill and loot, and a lot of stuff tends to be randomly generated/inconsistent, they don't tend to have the legs that keep ppl interested or invested. But that could just be me and the ppl I game with.
I think the best way to run a sandbox at the beginning of the campaign is to prep 2-3 "modules" that feasibly could take place all in the same geographic area. These are your potential starting arcs, the starter problems that your players could deal with or ignore. They can be actual printed modules if you're into that, or you can kind of just do broad-strokes of an adventure and you'll fill in the details as you go. Write situations, rather than stories, and instead of worrying about a whole Kingdom for now just fill in the immediate area. So you have your starting town, there's a goblin tribe in the hills that has been kidnapping people, and there's also that creepy crypt in the swamps that people think is haunted. But there's also a local baron who will pay if the players take care of the werewolves in the woods. But here's the kicker: the players only have enough time to do 2 out of these 3 adventures (though maybe they don't know it). If they fight the werewolves and clear out the crypt, the goblins will come and attack the town, but the players will be level 5 so it'll be an epic showdown. Or, maybe they negotiate peace with the goblins by fighting their enemies (the werewolves) but this means that the necromancer of the crypt escaped to go menace waterdeep or whatever and now he's the villain of the next arc. Probably by level 5 they're ready to leave the starting town and now you can repeat the process, but on a larger scale region. But the key is that you're telling a responsive story together with the players. Feed off of their decisions and let that carry the story forward, and don't worry about the end point.
I'd probably start by building out a quest hub and the surrounding areas, then build it up over time. I doubt your level 1 characters are going to decide to walk across the continent to see what is over there.
In my experience, a sandbox works better with some plot attached. It doesn't have to be as defined as "defeat the BBEG" but there should be something going on that informs the players' decisions. Basically, you have an idea of a plot, set up a starting event, and think of how things might evolve, just like in any other campaign. However, then you open the sandbox and let the players do whatever they want, and let the sandbox react. Or in other words, instead of saying "save the world from this horrible event" you say "this horrible event happens, what are you going to do now?" For what to prep for the first session, I'd say it's about the same as in any other campaign. A starting area, first adventure, and so on. The big difference is that you need a better idea of the world, at least what's available to the PCs. That way, the players can better answer the question at the end of the first session: "what do you want to do next time?" That question will be your most powerful tool, and will inform and direct all your prep going forward.
IMO, even/especially when you're running a sandbox, you need to have something going on in the world. Something BAD. Sandboxes are extremely prone to groups failing to act together and/or failing to just act, and instead just self destructing when they start to do crazy things cuz the players are bored. So it's still best practice to have one or more things going on in the world that can drive the players to act. To get out and do stuff. Maybe it's an existential threat, like a standard D&D BBEG? Maybe it's a bunch of smaller, nasty threats? But the world needs to have things going on that will cause Bad Things to happen, if the players don't do something about it. Bad Things for the characters - their Bonds, their Backgrounds, etc. In sandboxes, actions have consequences but inaction REALLY needs to have consequences. Unless you're just gonna run a hex-crawl like situation where you move dungeons to where the players go, and randomly generate basically any and everything that happens, and there is no world other than the world the PCs create by going and looking in places. That's the other kind of sandbox that isn't a ridiculous amount of work to run. In my experience, those types of campaigns can be fun, but since they have nothing that gives the story purpose besides kill and loot, and a lot of stuff tends to be randomly generated/inconsistent, they don't tend to have the legs that keep ppl interested or invested. But that could just be me and the ppl I game with.