there are many methods to do a colouring of a lineart :3 this style is the most "good looking" and HD one (if you want to sell your drawings for prints this one will be the best option cause it will not lose quality) its kinda simple, pick a basic colour for skin,hair and clothes and then just fill them at the areas that you want. About the shading at the start it will be kinda hard cause you will not be used to the light, after some practise you will be able to imagine the shadows and lighting spots much faster. If you want more tips about what style do you want just search for one image that you like and watch to colour like that and send me a pm so i will explain how he did that