In photo manipulation How do I know where I want to add light and shadow. Because light and shadow gives depth to the subject. I hope this is the correct question.
As to where: Decide where the light source is, and think about where the shadows and highlights should be placed, and maybe also some coloring. Look at photos and see how the light makes highlights and shadows in the real world. There's no other way to learn. Start practising with low contrast photos which don't have especially dark or bright areas.
Be aware that you can't make dark areas bright and colorful with acceptable quality (except by full manual repainting). It's impossible with ordinary photo files. Also, overexposed nearly white areas can't become colorful mid-tones with good contrast.
Use adjustment layers. "Curves" is a good method, because you can insert one curves layer for highlights and another for shadows.
Use fully black layer masks and spray white slowly on areas where the effect should occur. Fix by painting in black on the mask, if needed.
It's also possible to find the needed adjustments without layer masks.
You will also need coloring layers.
Sorry, but you should be more specific. Present a single limited scope problem to get better help.
To add shadow effect in Photo you can do the following steps:
1) Open your desired photo in Photoshop.
2) Click on the layer of the image to which you want to add shadow.
3) Click “Layers” in the menu bar then select “Duplicate Layer”.
4) Add different name to the duplicate layer.
5) Click on the “Layer Style” icon in duplicate layer. Now, click on drop shadow.
Make adjustment of shadow as per your requirement. Now, Shadow effect has been in photo.