You need some way to properly position the softbox "in space", like a short
boom arm with a counterweight. As shown, the softbox is right at the table height, and it is creating a big, lengthy shadow that comes forward, toward the camera position. Where the main light needs to be positioned "in space" depends on the individual photo and the desired objectives. Many food shots have some highlights that are actually reflections of the front of the main light, such as the sheen on top of a bowl of soup...that sheen, that diffuse reflection from say, the front of a flat-faced softbox that has been "flown over" the set, that is what gives the liquid its visual weight in the bowl...
This is the kind of stuff that the lighting book Tirediron mentions will discuss at great length.
I am going to give you the most valuable tip I can. You need to frame up the shot you want with mock-up food, or the real food, and have an assistant literally move the light through an arc, or two, or three arcs, while you look through the camera and continuously observe WHAT the light CREATES as it is moved. There is no substitute for moving the light through an arc, and observing what the heck happens, especially for a beginner. It takes a huge base of learning and practice to previsualize a lighting scheme, but with an assistant moving the light as you observe, you will quickly be able to evaluate light placement and correlate it with the effect it has on your food and plating. I can tell that you are struggling with some basics, such as why a flat,shiny table has a reflection from the softbox face.
Again--you absolutely NEED some method to hold the softbox in a variety of positions and orientations, over the set. You need a sturdy light stand, and a boom arm and its mounting assembly. You can rig a counterweight with tape and some cans of soup or bottles of water in a bag.You do not
need a 13 foot tall stand such as this one, but this is a good example of what is available at the lower end of the price ladder.
Impact Multiboom Light Stand and Reflector Holder - 13' (4m)