The thing about writing is that you have something to say, right? Right? If you don't know what it is that you're trying to say, neither will the reader. Stay focused on one particular thing rather than jumping all around, and you have a better chance of keeping your reader's attention. Think about it this way: when someone is telling you a story face to face and they start rambling side facts and little things that have nothing to do with the story they're telling, doesn't your mind just kind of start to wander...like their mind is doing? Which means you're no longer interested. Stay focused.
There are a lot of different ways you can keep a scene focused. The standard way is scene-sequel, well described here: Writing the Perfect Scene
I use the Story Grid to help my work stay focused. (If you can't afford the book, their podcast is wonderful: Story Grid Podcast.)
If neither of these work for me, I ask myself one simple question: What's my point? Basically, what am I trying to say in this scene? If you find your work wandering, ask: what am I trying to say? Just don't be surprised if you discover the answer is, I have no idea.