About to DM my first session, any advice?

As the title indicates, if everything turns out alright, in January I will be the DM of a short One-Shot for my friends and i need advices

It will be a simple session, because I don't feel capable of anything more complex; 4 scenarios at most.

The party will begin with a wedding between two nobles (I'll let them choose why they're there), they will start at level 3 so everyobody can have their subclass

After that they will go to a train station, where they will be told that they will have to take a train at night along with the rest of the guests across a huge cornfield. But strange things happen and appear in the night in that cursed place

The party's job will be to stay awake until 6 a.m., as soon as dawn breaks, protecting the train and its people from the strangeness of the camp

Once it strikes 6, the train reaches its destination and that's where I'll end the one-shot

This will be my first time as DM, and my 7th game session overall, and i'm nervous and need advice, for example wich enemies i could use, if i should have a boss before the end and all that