I'm proud of you for taking the step into digital painting. Don't be intimidated by all the amazing digital artworks you see and all the complicated shit on photoshop or whatever you're using.
Start by simply enjoying the brush tool, eraser, layers, and color, and as you keep doing digital artwork, you'll get to enjoy the slow process of learning a little more and more about photoshop each time you use it.
And I strongly recommend you use photoshop and get used to it, because it's the most broadly used painting program, and it has a lot of versatility.
In critiquing the artwork, I'd say it's quite flat. As you should practice coloring, it's also good to really squeeze your skills into making the best sketch possible. Take your sweet-ass time!!!!