Getting to the coloring stage of a piece can be a little scary or exciting depending on who you are. Personally I love the coloring stage but I have friends very much like yourself that stick to just the lineart. To put it simply, my process always begins with a flat color layer. I choose one flat color for each piece of the work and fill in broad areas with such. Then I begin the shading process.
Shading highly depends on the lighting situation in the scene you are creating. I would recommend doing light studies to get a better sense on how to actually shade an object. There are plenty of tutorials online for basic shading techniques as well. When it comes to graphite or charcoal drawings I use either my fingers or a tool called a tortillion if I don't want to get my hands dirty!
Painting or digital painting can be a bit trickier but like I mentioned before preforming various light studies will help tremendously! You can look up images online for "light study" or set something up in your house. You can grab a simple house hold object and place a light source nearby. Study how the light hits the object and how the shadows blend and contrast.
Each medium will have their own ways for adding color and shading. Practice makes perfect and through studies you will be able to gain the experience you need to feel more confident in this process! I do recommend also taking color theory courses or getting matched up with a tutor to help you out with it!