Critias, Walks through Walls and Stan all have wonderful advice. Heed them well.
I would like to touch on a few in depth details regarding adventure design, specifically as they relate to Stan's advice about making your own adventures at first. (These tips may not work for everyone, but they work for me, and I hope they help.)
1. Write up a rough outline of the adventure/story you want to tell. Think of it like a book with chapters, or even better - a movie with scenes.
Break up the outline into 3 parts. Beginning, Middle, and End. Or, to use SR parlance: The Setup, The Run, & The Aftermath.
Then, break each of those into scenes. For example:
The Setup
• Meet the Johnson.
• Talk to the contact that's in the know.
• Do some surveillance.
The Run
• Getting in.
• Accomplishing task 1.
• Task 2.
• Getting out.
The Aftermath
• Meet the Johnson again. Get paid.
• Tie up any (obvious) loose ends.
• Try to cover tracks/law low.
2. Think of a scene as a stand alone entity. This isn't a railroad. You don't want scenes that lead directly to the next one with no choices to be made by the players. But you do want them connected. Sometimes that is really easy, as the 4 items in "the run" section above are very cut and dry and linked together. But sometimes the run has a lot more to do with the setup section, as in the case of mystery style runs, where there is a lot more legwork and meetings and surveillance involved. Getting these scenes to tie in can be a little harder, but luckily they also tend to be a little more loose, so that makes it not too bad. For example, in "the run" in the above outline, "task 1" and "task 2" could be done in reverse order, so make sure you plan for that eventuality; keep them separate and independent.
3. Try to think of 2 different ways to introduce a scene to the players. A wonderful analogy for this item is the idea of drawing a map. We've all done it I'm sure. You draw up the forest with the elves in it, or the Ares Lab the players will be infiltrating, or the map of a region in a D&D game or some such. And in that map, you place the well, or the prototype weapon, or the evil warlords cave, respectively. Well, the problem with that is that now you have to figure out some way to get the players to the well (or the cave) so you can have your epic confrontation. You have to force them. Bad idea. Instead, try this: Plan the well encounter separate from its location. Make it a stand alone scene (a la #2) that is completely independent of location, obviously it needs to be in the forest, but where in the forest isn't all that important when you think about it. So... write up a small note (2 of them) about how exactly the scene could be introduced to the players, you want 2, in case one of them just plain wont work once the time comes.
4. ... and write up 3 (or 4) different ways for the scene to end. You need to make sure you have an idea where you want the scene to go. That's your first "exit strategy", plan it well. Then, for your second strategy, write up what happens if the players fail, or say no to the offer, or what have you. This is important because it helps you to think of ways to keep the players 'on track', with out forcing their hand, or railroading them. Don't try to make the outcome be the same for paths 1 and 2 though, you want them on a track, not on the track. And 3rd, stop for a second and take off your GM hat, forget about your story, and ignore all your plans. Then put on your Player hat, and look at the scene from their (pseudo-uninformed) perspective. What might they do in this situation? This is your #3 exit strategy. Sometimes there are 2 'player paths'. That's fine, write up a #4, but that's all. 4 is your limit.
5. When writing up a scene, keep it simple. By simple, what I mean is. You now have about 10 scenes for the run. Depending on how much time you have to plan, and how often you play, this can be daunting. So keep it simple. Don't plan out every single possibility or write up stats for every single NPC. Stay on target. Write up a short paragraph or list of ideas for the 6 senses (see below) to describe the scene itself. Have one or two sentences for NPS they are likely to run into. Have 3 or 4 sentences for the "major" NPCS, along with a few pertinent stats. And for anyone the team might actually fight, write up brief stat blocks of only the relevant combat stats for the person. Don't go overboard in your planning.
6. When writing up a scene, be descriptive. Use the 6 RPG senses. Sight, smell, touch, taste, sound and motion. The first 5 are self explanatory. Make sure to include at least 4 of them in your description of any place or person. The 6th is what makes a scene come alive. And remember: it's not "what is moving in the scene", it's "how is the scene moving?". When you look at any person or situation, there is a life to it. It's doing something. A person/crowd has intent, desire and a goal. A location/place/item has a feel, a purpose and a reason. If you clue the players in on what these things are, what the motion of the thing is, it will be real to them, rather than a static item/person.
7. When running a scene, don't be afraid to narrate. Don't over-control the characters, but don't be afraid to move a scene forward with actions that could be 'automatic'. Such as just after you describe a scene in a bar, you could say "after looking around the room, you spot your contact, sitting in the back booth. You access the bars menu and order a drink as you make your way past other bar-dwellers, getting bumped here and there by people as they cheer at the sports trids. You nod to your buddy as you sit across from him and as you begin small talk, your drink arrives via a cute dwarven barmaid." In this, you have taken control away from a player, but only in a small way, and in a way that adds to the description of the scene and moves the story forward. A good rule of thumb is to never 'assume actions' for a character that would involve a roll of the dice.
8. Plan your planning. Only assume that you will make it through about 4 scenes in an evening, 3 if you have short sessions. Only assume 5 if you have long sessions and/or they are very short scenes. And then, plan accordingly. There's no need to flesh out all the details of the final encounter of the run if you're just starting. Focus on whats going to happen next. AKA: plan in order, and try not to plan any further than one gaming session. A lot can happen in that one session, and you wouldn't want to have to go back and redesign 3 scenes that you pre-made because the PCs decided to do something you hadn't thought of last night during the bar scene.
9. Don't be afraid to leave a cliff hanger. You don't have to have every scene and every game night wrapped up in a nice tidy package with a bow at the end of the night. Don't be afraid to leave a scene a little open ended, and don't be afraid to end game night right before, right after, or even right in the middle of something big. AKA: it's ok to leave them begging for more.
I hope these tips help a little. Best wishes.
