Dungeon Masters narrate, control monsters and NPCs, design the story, and make sure that everyone is having fun and following the rules.
Dungeon Masters work to improve everyone's playing experience, sorting out arguements and helping to make compromises.
Preparing for a session can be overwhelming, as it can feel like there are an infinite number of possibilities.
When you are planning a session, you don't have to perfectly render and predict every single possible outcome.
You can set a general guideline, and then let the players guide where the story goes.
Be flexible, add some new things along the way, and remember, have fun!
There is one thing above all that signifies extreme danger, possibly even the party's doom. The smile.
If you are a Dungeon Master, and you want your players to be terrified for their characters' lives, (in a fun way) let an irresistable smirk spread across your face.
Maybe even laugh. It works every time*.
*Just don't use it very often or else it gets old.