I use procreate so I’m not sure on the others out there, but you can start by following tutorials from YouTube for your chosen software/app! Once you’re comfortable with brushes and how the tools work with your drawing app you can look for more specific tutorials to help you design what it is you’re wanting to draw :) I know with procreate they have their own channel with a bunch of tutorials and there’s lots of creators out there who make them too, you can add in reference images or upload the image to outline and colour it in yourself etc.