I want to create an adventure for my family, but I don't know where to start. Please post some tips!

I like to view things in three, so:

1) The setting: you must choose a setting for your adventure. It can be an official one, like the Forgotten Realms (shortest solution) or you can create one on your own (longest solution)

2) The story: you must create a story (or plot) which your players must resolve. It can be a villain or another kind of issue.

3) The adventure hook: it is the starting point for your player, which will let them to resolve your story.

These three points are described in the simplistic way possible, of course. Let know for more specifics.

I agree with filcat on this!

From how you've posted this, it sounds like you'll be running a one-shot, or a one-off adventure. Have a clear goal, a few different locations that will give them stuff to explore around, and a lot of plot hooks to draw them along the plot without making it feel like you're railroading them.

I think creating your own setting is the easiest option. You basically just need to know what Kingdom the PCs are in and what city they are in and a little about the surrounding area. You can improve everything else. Using a published setting usually requires you to know about the city your PCs are in and depending on their familiarity with the setting greatly influences how much you need to know. So I think its easier to just roll with stuff and improve than it is to remember everything you read about the city (which if you forget means you have to improve anyway). And with the small details current resources give, improving is probably going to happen anyway.

So all you really need to know is about the city/village/town the PCs are going to start in. If you are just starting to run, it should be a town or village, because those are small places that don't require a lot of thought. They probably have a temple and an inn and some farmland. They might not even have a blacksmith if its really small.

Come up with who runs that town.

So super quick town guide

1) Its Name and who runs it (helpful for hooks)
2) The inn/tavern details
3) Any place required for your hook and those the PCs are likely to want to visit/need to visit.
4) A few generic NPCs to inhabit the town

Then of course you need something for your PCs to do. Which could be anything. Add the story elements into the information you need. Assume if you plan for options 1 & 2, the PCs will chose option 3.

Make a compelling hook that drives the PCs emotions and investment into the adventure. You could say that the PCs start on a caravan and it gets attacked. But what if the Caravan is carrying the dead body of a childhood friend of the PCs and then it gets attacked? 

Lady Kalla,

To craft a good adventure takes a lot of work, so you have to be committed to spending the time necessary. I opened with that because you could just slapdash some scenes together and call it good but the adventure won't be good and the sad thing is you and your players may not even know that it isn't good. Now I am going to tell you some things that may cause others to lose their minds but what I will say is the truth. First, you don't need a setting to run an adventure, I know that sounds at odds with everything else you have been told but that is absolutely a fact. That doesn't mean your adventure won't have a setting, it just means that the setting is irrelevant. Basically, if you are creating a one-shot adventure then don't waste precious energy with a lot of details of the setting. Second an adventure only needs three things. The first and most important is the resolution. So many DMs get this wrong. You have to know where you are going in order to get anywhere. And you also have to know when the adventure is over. So basically create the resolution first. Then jump to the motivation. You have to present a reason for the characters to want to go on your adventure. But here is a dirty little secret, the motivation doesn't have to be that great because the Players already want to play. However, a strong motivation will just enhance the feel of the adventure. After you have the motivation then craft the opening scene. Now it is important to nail this because a strong opening scene can make or break an adventure. Just like a great movie or book the opening captures the attention of your audience, which happen to also be your protagonist, and makes the audience care about what happens to the heroes. The craft the climax, this must bring your characters to the resolution of the adventure and let the players know, without a doubt if they were successful or not (Yes, your players can fail, but that is a different discussion.) Finally, fill in the middle with a bunch of scenes that lead the characters from the motivation to the resolution. And that is all there is too it.

One of the biggest tips I think I use is encouraging the players to make the story for you. The back story of the characters can give you ideas for villians, guilds, locations, side quests and more. I have one player, a fiend hunting ranger, who got her start saving her father from a rakshasa with a stroke of luck, igniting her adventuring soul and plunging her into hunting the infernal. That rakshasa is gonna be a recurring miniboss here soon, seking reven get agains her and her father. Another player is a runaway noble that faked his own death and became a druid to keep from dissapointing his family due to not wanting to take the title. However, his family is being manipulated by a secretive cult the party's been hunting, and he'll have to go home in disguise to save them without outing himself. The more your player's can provide about their characters, the more the story will jump out at you.

Even if they aren't little writers, you can get ideas based on the stuff they look for or look to do. I have a player, Dwarven bard, that was miffed that the music shop he found only had the typical kinds of instruments, complaining that he wanted something plus one or unique. Now, i'm trying to work out how to include a hunt for a special instrument into the campaign. Everyone wants something, and dangling that something in front of them may be as much as they need.

I'm assuming that you are ALL new to D&D, so the best advice I can give: start small, use common tropes, and begin with a railroad. 

They all begin in the same small town. It doesn't REALLY matter how they all got there, but if that matters to you, have them figure it out. While they are creating their characters, tell them that they will be starting in a small town and to figure out how/why they are there. Maybe they live there, maybe they have a friend or a family member from that village, maybe they were travelling there for work, whatever the case may be. That's up to them. While they are at it, maybe have them figure out if they already know each other or if they will meet at the town. 

Then start with combat: "You awaken to the sound of screaming as smoke begins entering through the open window. Upon throwing open the curtains, you see the local <insert building here> ablaze..." 

Once the players arrive outside, a fight with goblins/kobolds/bandits/etc... ensues. It was they who started the fire, but this was merely a distraction. After the enemy is defeated and the fire put out, a villager comes running to the group panicked exclaiming that their child is missing (despite the obvious trope, kidnapped children are as good a hook as any to get a group of good-aligned party members to take action).

This will lead to a good time for you to introduce skill checks to the game to track the enemies down (though should all else fail, simply talking to someone in town should net a "I think they went THAT way"). 

On the way to the first "dungeon" they should have a "random" encounter (roll behind the screen a couple times, but don't pay attention to the results, just have it happen after a couple of rolls); whether it be area appropriate beasts that they can either fight or avoid, a helpful caravan that says they saw the enemies heading in a certain direction, or a cell of the enemies on patrol, so that they learn to expect random encounters in the wilderness. 

Then have your first dungeon (cave for kobold/goblins/etc... or a hideout for bandits). Start with a few scouts at the entrance keeping lookout (relatively easy encounter). After one of the scouts is reduced to 1/2 health, make it so that one of them runs away to warn the next group (this will let them know that sometimes the enemies will run away if they think it would benefit them to do so - these are semi-smart, but low level enemies, so they are likely to act cowardly when up against a superior force). Only have this "dungeon" have a few rooms, because lower level characters have relatively low HP and resources. And have the final encounter be with a leader of some sort. After defeating the leader, you can either have him exposition something pertinent to your campaign or have the players find a note written to/from a more powerful entity that the leader was working for/in contact with. This is a good way to set up your big bad evil guy or one of the lieutenants of the BBEG. So that when they encounter the character later, they have a "hey he's evil" reference point. The kidnapped person was either for ransom/blackmail purposes or for use with a ritual of some sort (children/virgin blood is always a catalyst for evil rituals). 

Lady Kalla, start with Matt Colville's Running the Game Series, and you won't be led astray!