The first step Is drawing your image on paper, they all do this, then scanning it and bringing into Photoshop.
Most professionals have some kind of tablet they use to clean up and make lines darker on the computer.
(note that they don't usually draw from scratch using the tablet unless they are using an on-screen tablet)
VS
Once you have your lines clean and pretty, you just need to create a single layer under your lines, and start painting with your tablet.
*Use layers ABOVE your color layer for highlights, and put shadows BELOW your HIGHLIGHTS layer.


*Just a suggestion, not anything you have to follow