Start with Krita, don't waste time. It's made specifically for digital painting and has a much smoother learning curve than GIMP. The interface is intuitive, the brushes are decent right out of the box, and it has everything you need to get started without overwhelming you with options you won't use. It's not full of weird buttons in random places like GIMP tends to be.
GIMP is more versatile and you can do almost anything with it, but it's basically a photo editing tool that can also do painting. If you're looking for pure digital painting, you're going to get frustrated with confusing menus and workflows that weren't designed for drawing. Krita was built from scratch with artists in mind, and it shows.
Try Krita first. If you then feel like you need different features or more control over certain aspects of editing, then give GIMP a shot. But honestly, most people who start out do just fine with Krita for a long time. It's free, has an active community, and there are tons of tutorials all over the internet.