If you have set it to take jpg files, then the camera will be processing its output as its programmed brain sees fit. If you open such a file in Pshop, then the baseline might be to check out tonal range and distribution using the curves and levels dialogues. But really, the first step is to lern to see how light falls on things and how they receive it, and understand how the camera meters the exposure for what it's pointing at, and learn when you need to intervene and alter its settings. Then its your vision from there on.
Look at layers, masks, blending, filters ...