well I'm actually a beginner too believe it or not I've only been making tiles for a couple months ..
but basically you make a checkerboard of 16 x 16 squares becuase 16 x 16 is one tile the colors dont matter as long as they have contrast. the you decide what your making, becuase buildings you would do differebt than say trees. For buildings you make the outline on the grid and make sure the door is the same height as your selected OW size, then you can make sure the building size is proportionate to the door. After the outlines done I do flat color and then just start shading, the style of tiles depends on how you shade. For trees and rocks i make the shape out of the darkest shade and then I layer the lighter shades on till it fills out and looks good. Sorry I cant explain more but like i said I'm new to it too