It's not just using a calculator, I've never used anything like that. You need to learn more about how your camera works, and how to make adjustments in aperture and shutter speed that work in a variety of settings and with various subjects in various lighting conditions.
Good idea to back up the bus and go learn how to use the camera well. And practice a lot. And later on maybe you'd be good enough at it to do shoots for other people. (And people will apparently take just about any pictures they get for free by talking somebody with a camera into doing it, lol - and it seems you can't go just by friends & family who might just be trying to be nice and supportive.)