Umm I'm pretty sure I'm either doing digital art exteremely wrong or just have a bad app

I'm using Pixelmator and an iPad and I don't know how to use them correctly 😂

Can any of you digital artists help me and give me tips like how to use layers and stuff???

I wanna do like shading and stuff and AM I SUPPOSED TO COLOR THINGS IN MANUALLY OR IS AN AUTO FILL USED???
hALP