D&D Advice for Starting DMs?
"The facts of life..." -Dr. Tyrell, Bladerunner
OK, first, if at all possible read up on the subject in the DM Guides. Plural. Anything from 3.5 and up, as a matter of fact, has valuable insights on the how-tos. If you could browse through the 3.5, 4 (yes, even 4) and 5 dungeon master''s guides, you'll get an evolving view of what works and what doesn't.
If you have no time, and need quick-n-dirty advice, here goes:
Map out a list of the powerful characters in your World: the Kings, Dukes, Archmagi, Ancient Dragons, down to the mayors and crime lords in the towns you're likely to be running. Name the organizations. The Free Association of Fun and Frolic has a nice ring to it, even though it still might mean "Thieves' Guild"...or it could be the Guild of Tavern Owners...or it could be both. Or they could be separate and competing. But you've got to record it somewhere, because it will become part of the story.
Make sure you note their motivations: long term, short term and immediate. "The All-Squirrel noted a 15% drop in nut production this year and is concerned by the predicted 20% population loss."
Come up with a list of secrets in your world. The weird subtle ones. The squirrels are all part of a hive-mind All-Squirrel. The Dragons are slowly disappearing. Magic is stronger around mountains and sources of certain metallic ores. Running water disperses necromantic energies (a classic). Clouds are sentient, telepathic, and love smoke plumes...and no one knows this. The elves are preparing for a war next century, but don't know exactly who against. The dwarven society has a thriving warlock community that dominates the public pantheons from behind the scenes...and they have a plan.
Who is chasing what and why will give you a ton of story...and side-stories...for years.
Also be prepared to shitcan all of that prepwork when the players do something completely unexpected to derail everything.