First time dm

So i am a first time dm, and i wanna run a one-shot that preludes the campaign. but i'm having trouble fleshing out the one-shot. the basic premise is that the party is level 10, and they need to investigate an ancient dragon by order of the cobalt archive equivalent in my campaign. the way i wrote it now is that the dragon is being mindcontrolled by something, but i have no idea what/how. the players are all also first time players (i've been playing for a while), so they'll also need some help in making their characters too. any tips or advice?

One big piece of advice is not to start first time players at level 10. There’s just way too much their characters can do at that point, and they’ll be very confused. First timers should really start at level 1. Let them gradually add powers as they go.

Agree, I would definitely recommend against starting new players higher than maybe level 2, but you should really start them at level 1. There's SO MANY abilities at level 10, not to mention all the choices that go into character creation to get to that point - if they don't really know what they're building it's hard to make choices they'll be happy with. Plus it'll be very difficult to actually run these characters effectively without them getting incredibly frustrated trying to figure out what to do. Consider toning your one shot down to something far easier so that you can learn to DM/they can learn to play the game.