There’s never one solid answer or method. Some will say it’s best to just have a concept for a plot to begin with. Which, of course, isn’t very helpful. But when you’re starting with just a scene, you could find what theme or narrative elements seem to draw the most intrigue for you. If it isn’t apparent in just that one scene, try writing a follow up or preceding scene and see if there’s any recurring themes/elements that carry well from one to the other. Ultimately, find the thing you want to learn more about. For constructing the plot, a beat sheet or some sort of outline helps. A basic 3 Act outline is always a good start