I'm a newbie Krita user, I'm trying out digital painting out of curiosity and would like to seek help on how I could make shadows more realistic using mouse (I've got no drawing tablet)

Here is the link to what I'm working on https://s-media-cache-ak0.pinimg.com/originals/7a/5c/0f/7a5c0f42071e42609b42e3f4a7c7a942.png

any tips or tricks would be appreciated.

I highly recommend do not draw or paint with mouse. It won't be good, I've tried. The best you can do with mouse is good vector lineart on top of pen\pencil scan (but I think Krita isn't the best solution for that case).

Based on your wip I guess you are newbie in any art, not just digital. So another advise: stick with pencil and paper for first time, and buy a tablet later, then you'll know for sure that you won't lost interest in art.

Anyway, the easiest way to add shadows is probably "multiply" layer. Create new layer on top, change mode to "multiply" and try it out.