If you are practicing realism, you might want to study how light interacts with certain objects. It's not that easy and accuracy is really hard to achieve, that's why you need to study and practice more on painting lights and shades on your artwork. Here I will demonstrate the basic ways on how lights interact with simple objects like circle and other shapes. Even me, I still have hard time figuring out if it's close to realism or not.
I also encourage you to try on other objects like cubes and try to add some textures on it.