Start with the AV mode to control your aperture, then watch your shutter speeds automatically change according to the amount of available light... this should give you an understanding for the what and why (when you are ready to go full M mode). The fact that you distress about your settings changing constantly to your situation, leads me to believe you have a lot to learn. A photo shoot location is in by no means controlled, it is adapted to...even using a studio light setup still requires adaptation and changing of your camera settings (you reposition lights/subject... new settings).

It is important to just use your camera to experience/figure out what is going on with the results. Read about equipment to see what it does and how, you are going to have to invest some time.