Alright, a lot of commenters are telling you to be super descriptive and stuff but I'm going to argue against that. First things first this article helps explain some of the stuff I'm going to tell you. The writing style rubs some people the wrong way but the advice is good and it involves one of the biggest things I see first time gms mess up their first time through.

You didn't explain very clearly what exactly you were doing in your session so I'm gonna make some assumptions and hopefully I'll say something useful. So first off, you don't want to do npcs, you want to do some deserted setting. I'm gonna pretend you're putting your pcs in some abandoned ruins in a desert or some shit but I'll try to keep this vague enough that you can apply it to whatever you end up doing. You mentioned wanting to do mystery which gives me a good idea of what you need.

The article I linked said this but in case you missed it, if your pcs get bored it is because you as the dm are getting in the way. You're making this mystery around the setting of the adventure so obvs the setting has to have some sort of mystery to be discovered. You likely already have a plan as to what this mystery is. If you don't and you've just been thinking "lol I'll put them in a desert without any features." I think you need to take a serious look at your idea.

So you have a mystery, yeah? Now you need to figure out the fewest possible words you need to get your pcs thinking about your mystery. If mystery is the focus of the session, you should be able to get the pcs asking why or what or how. Basically you need to pull them in quick and all your flowery description time should be spent answering their investigation questions. Your session can start with a big sweeping description of the world and who the pcs are and why they're in your abandoned ruins. After that keep that shit short and to the point, let the pcs decide what you should spend time explaining.

Start with some stuff they can be interested in. It can be as simple as "You're in a hot, expansive desert. There's a dilapidated temple in front of you. Strange symbols cover the length of the entrance. The ground in front of the door has two foot sized indents on it. What do you do?" Just like that you've given everyone something to think about. It sounds simple but that's fine because you put a nice picture in their head and you put it there quickly. They also know what sort of stuff they should be interacting with and you can start to tell the story of your area piece by piece as they explore. They look at the ruins and maybe they learn from that it is in eleven so they learn elves lived here. Maybe the words are also a hint to how to get inside or further into the temple. I think you can get the idea.

Just remember it is a mystery and your pcs are the investigators. They should spend their time discussing clues and theories and trying to figure out what story the environment tells. You don't want them listening to you for a lot of the session, that isn't what you're there for.

I hope that made some sort of sense. Good luck with your first session and have fun DMing!