What's a good way to structure a dungeons and dragons campaign?

Learn to improvise, and have some generic encounters prepared if the players decide to pick a fight with guards or they're just getting bored on the road and need something to spice things up.

Make it up as you go along, within reason

>only squatting 1pl8
>in a fashion that would make make gainz irrelevant because you would snap your shit up within seconds
this is what happens when you normalize crossfit

Depends on if you want

1. Dungeoncrawling
2. Open world
3. Social
4. Everything

Focus

Dungeoncrawling owes better to preparing a dungeon beforehand
Open world and social owe more to making some simple baseline structures and keeping a few backup generic encounters and scenes you can fit in anywhere and then improvizing the rest

Another question: How would I give them a reason to get invested, like I work on a small quest, but I want to give players a reason as to why they should do something. And also maybe some ideas as to why they'd be adventuring together.

Railroad them into motivation. if the quest is X must do Z then make Y create some of the motion.
Wealth is a classic reason to be adventuring, but the reason your group might be going out is definitely setting driven

Here is a good way to structure plots:
thealexandrian.net/wordpress/7949/roleplaying-games/node-based-scenario-design-part-1-the-plotted-approach

Work with the players on making their backstories, not just bringing characters to the table day one. Integrate them into the world, clarify how their character interacts with what you do.

Putting players together can be done many ways, mutual gain and avoiding mutual peril are the standbys.

Say they are in an inn, which starts out with normal conversation between their characters who find themselves at the last empty spots together on feast day. The bar is lively, with wine and music flowing in equal measure. But wait, they notice a pair of thugs hustle a scared barkeep into their rooms upstairs!

If they go to help, then play out straight combat, listening at the door, or potentially a hostage situation with these thugs. Perhaps they make powerful enemies of the thieves guild? Give them a chance to shine here, it is their first time together after all, this is about defining how they react to situations.

Say they don't help him, but proceed to loot the bar in the commotion of revelry. Maybe the thugs then frame them for murder when the city watch comes in. Maybe the city watch is working with the thugs, or like a gang all their own, drawing the players into a mutual conflict.

Give them a chance to shine, and an intriguing glimpse of what could be next. There should be many reasons to be interested, both for them and their characters.

This is why building backstories collectively is best, because it lets you see what they are interested in, what would get them hooked.

Throw something shiny on the table. Enjoy the show the players put on for you.

This seems like a ridiculously bad idea, what is he doing?

Here's the problem:
The players want their actions to matter somewhat.
The players also won't pick up on much or offer their own ideas that often, especially at the start.

So, if you want things to happen, you have to make the events you want to happen fit what is going on. Sometimes called "Quantum Ogres."
You should have a plan, but be ready for the players to not follow that plan. Requiring a fair bit of improve skills, or at least flexibility.

So, have a big end goal. And slowly work from where the party is, to where you want them to be.

That way they don't just run from trying to stop a lich to overthrowing a kingdom in the name of gay marriage...

not even on the other boards we're safe from the horrors of crossfit

Depends on the version.
Not a joke, it really does change flow wise, specially on the following AD&D (AKA pre 3) 3, 3.5 and 5E and 4E.

First off discuss with the players what type of characters and game they want to play and their characters' goals.
Next, decide what sort of game YOU want to play while tying together the players' wants and goals. Long-term plans and themes equivalent to the player's goals are good with the understanding that the details can change to fit the game.

You begin the adventure with whatever circumstances to have the party meet and unite depending on the characters and their goals. Maybe start with some quests with rewards the characters want to advance their goals while dropping hints of larger plotlines including whatever overarching main plot that you want.
From that point on just play it by how things go.

Squats.

Read the goddamn DMG or the DMG 2 for 3.5 DnD.
Full of good tips.
You need to read to enjoy this hobby, user.

thealexandrian has some fantastic articles for running games.

Their 5 room dungeon and how to run a mystery session are great.