SIlverClouds18

I feel soo silly as you can see i'm a complete newbie....but thank you, thank you, thank you soooooooooooooo much !!!!!!!!!!!!!!!!!!!!!!!

Please can someone help me, i've just finished drawing my first ever drawing on Paint.net, It's of a a horse from scratch, but know i'm having trouble with shading, i need tips on how to make the shading realistic, addin darker and ligh colours, i've tried a lot to different ways, but they all look extremely messy and horrible, is the a neat and proffesinal way? PLEASE HELP!