Lots of good advice. There's a few things I'd like to add to what's already been said based on my own experiences.
1) Focus on Stories, Not Stats - Sometimes we have something too specific in mind and it limits us or screws us over. In a long running campaign I planned on the PCs taking on the evil, tyrant necromancer around 8th level. That never happened. I didn't plan on having a campaign that lasted 5 years. The campaign took a turn for the better and we had a lot more fun doing other things. The stories of the necromancer were what was important to the setting, and not the stats. My original idea was a basic Necromancer NPC with a few magic items and lots of minions but it was never set in stone. The PCs never actually faced the Necromancer until they were 20th level. By then, after research, spies, and scrying, they discovered the evil tyrant was a Death Tyrant from outer space whose ship crashed aons ago. That wasn't my original plan. If they would have faced a CR8 monster at 20th level it would have been a complete joke. If I would have planned out the encounter, rather than focus on a narrative, I would have completely wasted my time. Build the stats when the encounters are closer to happening.
2) Don't Look too Far Ahead - There's no point in worrying about what the PCs will be doing when they're 15th level if you haven't even started your first session. Trying to plan that far ahead will overwhelm anyone. It's good to have a meta-plot for your campaign. However, focus on the moment, where the PCs are right now. Then try to build encounters and a narrative that fits within the framework of your meta-plot while not being contrived. From my own campaign, I had the PCs working as Road Wardens to keep travel safe after the Great War. My meta-plot was a Spelljammer game with all kinds of strange aliens. So rather than BOOM! there are aliens everywhere, I started it as a normal D&D game to set the tone. It starts off like normal D&D where there are Zombies eating brains, and the PCs are fighting the remnants of an Undead Army. Then it turned into the horror trope where they find dead bodies with their brains sucked out. First instinct it's more Zombies, but where are they? It's actually Aliens eating brains, and it quickly turns into a sci-fi horror. How long did it take to get there? About five sessions. Basically it took as long as it needed to establish the PCs and their place in the world, and then turn it upside down. That's what the others meant when they say "don't try to control everything." You need to have a rough plan, a story, but then let the players and their characters move around. Take notes, and revise. See what they're doing and revise. I planned on three sessions but we were having fun being Road Wardens so I just let it happen.
3) Helpful but Kinda Useless - Adding NPCs that are friendly or helpful, who have their own motivations, is a great way to bring the world to life and provide exposition without reading boxed text. I tend to make them useless in some way: too scared, too busy, frail, incompetent, unreliable, etc. In your campaign it would be easy to have the players wake up and a friendly, very old, fisherman is standing over them. He's old so he doesn't have good eyesight, and can't really tell them what he saw or heard. He pulled them up in his nets and by the grace of the Sea God they're alive. He can tell them all about the town, and point them to the harbormaster for more information about that Caravelle they see out there. I add lots of NPCs like this in my games. Characters like this are perfect for delivering the plot hooks without feeling like it's a train ride. They're also useless for adventuring, which means the PCs are the only ones who can remedy the situation and must take action themselves.
4) Session Outlines - Years ago someone posted a session outline format on this forum and I've been using it ever since. I tried to find it with a search but I can't, so I'm sorry but no link to that post. However, a session outline helps me stay focused and gives me the basics so the game is running smoothly. Writing a session outline is pretty simple. Here's basic stuff from my own outlines:
- Quick Recap - Start with something you can read to the group to remind them and yourself what happened. "Where we last left off the Road Warden heroes had just hunted down and defeated the orc raiders when they witnessed a great fireball fall from the sky and land in the town of Hillsfoot far off on the horizon. Riding their horses, the heroes move swiftly to investigate." A few simple lines to remind everyone what happened and to get ready for what's about to happen next.
- DM Goals - I usually have few simple things I want to accomplish in this session like "Evoke a feeling of paranoia and isolation" and "Reveal horrific alien origins for previously mundane fantasy events".
- PC Goals - A few things the PCs are trying to accomplish like "Investigate the falling star" and "Protect the people of Hillsfoot". These are often more important than the DM's goals because the players often forget why they're doing what they're doing.
- Locations - Jot down a few places of importance and a few sentences outlining what that place is, who's there, and why it's important. I'll usually have 5 locations in my outline, something to read quickly so I can create the scene for the players. Oldham's Farmhouse: the farmhouse on the hill belongs to the Oldham family. The front door is wide open. There are no lights on in the house. The PCs look around and find no signs of forced entry. On the 2nd floor, Farmer Oldham is lying in bed. He's very pale. Closer inspection reveals a blood-soaked pillow and a hole in his head. His brains are missing.
- Encounters - A few encounters I want to happen, and keep it simple: Monster (CR and exp), quick description, motivations, and treasure. Psychic Ooze (CR1/2; 100exp; pg 240, 243 MM); The Psychic Gray Ooze is devouring townsfolk. It's broadcasting echoes of the last victim's thoughts, and luring people in. Treasure: Boots of the North. You hear a familiar voice. The sheriff is calling out from the barn. His voice is muffled. "Help! I've fallen and I'm hurt. Is there anyone out there?" You look into the barn and see a big puddle, and the sheriff's boots in the middle of it, glistening. Roll initiative!
- Random Treasure - I'll put some thematic treasure in there and have a random table, usually 1 to 6 and keep it simple. If it's pirates it might be coins, scrimshaw, jewelry, potion of swimming, potion of water breathing, etc. These are where the consumables go. That way I don't have to worry about what I'm giving them and letting the players roll for treasure is always fun. It's like gambling, but you always win something.
I save my outlines on a Google Drive so I can access using my phone. Over time I started typing up my entire adventure online, and then referencing it in my outlines so I can access the full notes of the adventures whenever I need to. Keep it simple and thematic so it's easy to read quickly and improvise.
Edit: a quick story about DM anxiety. Around 14 years ago I was running games for the neighborhood parents and the kids. My friend's brother-in-law hadn't played since high school and was eager to start playing D&D again. We had 11 people playing and most were kids. I felt completely overwhelmed, like nothing is getting done and I'm herding cats. So I went home defeated. Then I started getting messages on our Yahoo board from the parents and my friend. Everyone loved it. What I thought was a disaster was a lot of fun. So I read carefully and came back with a renewed vigor. I ran the sessions more "tournament style" because there was so many people: each person has 30 seconds to complete their actions, roll all of your dice at the same time, only read out the final result, etc. It was even better then next session. After the summer was over the the kids were back in school the brother-in-law wanted to run a game. I'll never forget that first session. He was shaking as he's reading his notes. He was overwhelmed with fear. Our friend just put his hand on his shoulder, "Slow down. Take a breath. Start over." He had to overcome his fear of public speaking, fear of failure, and basic DM anxiety all at the same time. Let me just say this guy had run some of the best campaigns I've ever played in, including that first one. His campaigns were brilliant in their own way, and I've stolen a LOT of great ideas from them. Fast forward a decade and his son is getting married. He's standing in front of 80 people with a microphone, no problem. Being a DM helped him overcome his fear of public speaking. You just never know.
