I am very new to D&D, I have maybe played 20 sessions over the last 8 months. My friend always DMs his own campaign and I thought I could try DMing a one-shot so he can play. 

My idea is a Scottish highlands setting. Maybe make a few bare-bones backstories for the PC they can customise them slightly. I have a few ideas for an actual story but they are pretty basic imo and I don't know what I can do to flesh them out.

1. Somehow the PCs get accused of being a part of a gang that steals some things in a market, bringing them all together in a cell and try and escape

2. Nearby village poisoned and people are dropping like flies. Surrounding clans pull a few of their best folk together to find the root of the problem and put an end to the mysterious deaths. Maybe its evils monks making poisoned wine? 

3. Similar to the last one but mysterious deaths, a big monster is destroying villages. Fight against Loch Ness Monster.

I voted evil monks but the Nessie idea could be really cool, just make sure you find some more combats to flesh it out!

The escape will be tough if you’re inexperienced... and potentially boring for the players who feel trapped in their options. And then beyond the escape after a couple of sessions, then what...? An escape from the law...? It’s a fun story idea, but really tough to pull off. 
if you instead made a prison story - Shawshank style - where there are gangs inside the prison, corrupt prison guards, failed escape attempts, evil necromancy tests by the warden... and you spent 10 sessions in the prison until they escaped... would certainly also be challenging, but then at least you can relax a little on getting the pace and tension right. You’ll have some breathing room as you establish as much of the prison as possible.

definitely check in with your players what kind of game they’re expecting.

murder mystery is fun. Yet again depends on players. You’ll need a whole bunch of social encounters to make all but the face of the party feel that they are contributing. Planting plausible logical clues is tricky so don’t wing that... map that put meticulously. Work backwards. Frustrated players are prone to beat up and torture witnesses so either allow for that eventuality or mitigate it by strict ethics of deputized investigation.  These stories need red herrings, false leads, and a final twist.  Watch some old murder mystery movies (name of the rose comes to mind) for inspiration.

fight the Loch Ness monster.  Cool idea.  Depending on length of session or adventure, you can really build the dread up slowly here. If you think of it like Jaws, we need to brutally and indiscriminately kill people away from the heroes, with them arriving on the scene afterwards. Provide lots of clues the nature of the beast, but also throw in red herrings too... scavengers have picked at the corpse, a lantern was knocked over burning the bit mark, there are strange flat marks in the mud that have been disturbed by horse and carriage. Have upset villages go on their own anti wolf hunting party in the forests to throw off the scent. Have wizened old drones talk about the gloom stalker, a scythe wielding death dealing maniac that hasn’t been back for 13 years. You just want to add horror and dread and muddy the waters so that they don’t turn to you first scene and say “we search the lake”.  a scene on a very small boat with a very large black shape swimming beneath it when all clues do lead to the lake. End in a final confrontation in an underwater cave. This is a story about mounting horror and maybe preparation... did they manage to bring fire down to the cavern? How are they going to kill something that big and strong? [erhaps they need to build a trap when they see the size of it.  Just wailing on it with clubs and swords might be a little anticlimactic. 

Wow, thanks for the response! I hadn't put much thought into the Loch Ness monster part yet. These are all great suggestions!! I wasn't sure how to go about a hunt for a massive creature but I like the false hunting, I suppose they could go and kill a low-level werewolf or something?

I really wasn't sure where to go with a prison break one-shot but I had the idea that the reason they were in there was they were mistaken for part of the raid and they want to confront the leader of the raiders or something? Idk  

its more interesting to let them tell you why they are in there and less interesting if they just hear it narrated... especially if you begin describing what individuals did wrong to get them caught because you have removed their agency from any of those decisions that got them into this hot mess. So just flip it. Say "One way or another it all went down so, so wrong and now you're behind bars. It really shouldn't have happened, but it did. Tell me, what happened to get you in this space?" 
If they're meant to have been caught at the same time... then have them come up with what went wrong at the same time. Make sure everyone gets a word in to describe some aspect, don't just let those with a big mouth tell it for everyone else either. 
If they're meant to have been caught individually at different times, then this can be them sitting together at the same lunch table roleplaying the conversation: What are you in for?

You might have an idea of some thing you wanted to have happened; a double cross, a stolen jewel returned, a framed innocent man, a murder... listen to your players and suggest these things in their narratives to add to them, not replace them. 

Quote from Torquemadaza >>

The escape will be tough if you’re inexperienced... and potentially boring for the players who feel trapped in their options. And then beyond the escape after a couple of sessions, then what...? An escape from the law...? It’s a fun story idea, but really tough to pull off. 
if you instead made a prison story - Shawshank style - where there are gangs inside the prison, corrupt prison guards, failed escape attempts, evil necromancy tests by the warden... and you spent 10 sessions in the prison until they escaped... would certainly also be challenging, but then at least you can relax a little on getting the pace and tension right. You’ll have some breathing room as you establish as much of the prison as possible.

 

Clearly I need to introduce you to this:

Thanks for the ear worm that is their theme song! hehe.

Maybe for option 1, they are trapped on a plane such as Carceri?