I've said this before in this forum I'll say it again here...
Determining shutter speeds and apertures for any given image is a balancing act. You're playing with two different gas and brake pedals that determine how much light hits the sensor during any given exposure. Shutter speed is a time function while aperture changes the diameter of the opening thus controlling the volume of light. Seems simple, but changing either function has other consequences. With shutter speed, as you increase the duration that the shutter is open with slower speeds you lose the ability to freeze moving objects. The speed of the object determines how fast a shutter speed is needed to stop it. At some point you will also lose the ability to prevent the motion of your own body from creating blur in the image if you're shooting handheld. For me this tends to be at about 1/3 of second with image stabilization on a good day. YMMV With aperture, you're physically altering the size that the iris in your lens opens during each exposure, thus it controls the physical volume of light that can pass through the lens to the sensor. Changes in aperture have the wonderful side effect of altering the depth of field that is in focus. As you decrease the aperture (smaller opening but larger numbers, stupid but that's just the way it is, learn your f-stops) the depth of field grows larger and vice versa.
To understand what aperture and shutter speed you want to use for any given image you need to fully understand what effect a change to either or both will have on the image you're trying to create. The best way to do this is to grab your camera and sit down at you kitchen table with a beer. Put that beer in the middle of the table and start shooting. Start with the smallest aperture your lens will allow, for arguments sake let's say it's a 50mm f/2.8 lens, that way there's no variable aperture and no change in focal length. At f2.8 take a couple of test shot and set the shutter speed so that you're getting a good exposure. Now leave the shutter speed alone and take a test shot at each full stop through f/22. Then go back to f/2.8 and take take the shutter speed all the way to the longest setting it will allow, probably 30 seconds. Take another shot a 5 seconds, 1/3, 1/10. 1/100, 1/500, 1/1000, 1/2000 and so on depending upon how much ambient light it takes to get a black frame. DL the images into whatever software you use to view images and look a the subtle changes between each adjustment. Sounds like a shit ton of work but in reality you can knock it out in 10 minutes or less as we're not winning any awards with the PBR can on your table.
Hopefully this will illustrate why it matters to be in control of each variable as even subtle changes that the camera might make in milliseconds will effect the image you end up with. The result being an image that was based upon some averaging algorithm and not on the desire to create a certain image. With practice this becomes second nature and you will be able to make subtle and rapid changes without ever taking your eye from the viewfinder. More over, you'll be shooting the images you want and eventually understand how to push the limits of each to create more dramatic and better imagery. Just because your image is in focus and exposed properly doesn't make it a good image. The camera is just a tool, learn to use the tool and you'll be amazed at how much better your photography will get.