Really generally speaking, you want to be in a position where you have better options than your opponent does. How you can use the layout of the stage to accomplish this varies by match-up and stage, unsurprisingly. Still, there are a few really common good situations.
For example, it's really good to occupy the center of the stage. From here, you can often approach without putting yourself at too much risk and you can retreat pretty far should you need to. It is also worth mentioning that you shouldn't sacrifice the center of the stage easily; It can often be tempting to chase somebody onto a side platform, and while this is often fine, messing up can allow your opponent to get below you or get to the center of the stage. This doesn't mean you should perpetually try to occupy center stage, or even that you should be spending the majority of your time there, but it's often good to try to stay closer to the center than your opponent is.
Other positions that are situationally good to occupy include the space below a platform, since you have the option of wavelanding onto the platform, whereas you don't have that sort of movement option elsewhere on the stage, and the top platform, which some characters have a hard time reaching.
I don't think it's necessary to think of stage control in this sort of way to get good at it, but it definitely helps; I know that I wasn't any good at it until another player in my area whom I used to regularly practice with pointed out how I could be positioning myself better.
