Definitely give Krita a try - its free to download, which is a massive plus. Its also quite powerful for the price of nothing, and I freaking love it. Its very much catered towards digital art so it lacks a lot of the photo editing techiques that you'd find in Photoshop and Gimp, but makes up for it with the huge amount of things you can do with the brushes. I'd recommend having a look if you're looking for a free program to work with. Just, if you want to add text by typing, you may want to transfer the image to another program. Its painfully annoying to use, but that's the only drawback I've found so far. If you do use it, there are plenty of tutorials on youtube to give you an idea of what its capable of.
Working with a mouse is hard, but completely doable. I did digital art for a few years using a mousepad and a mouse, and there are many people out there who do some amazing stuff with them. They're just a bit finicky to use at first, so if you have the budget for it you might want a pressure sensitive tablet, but if not, don't worry too much about it. Same goes for a scanner - it can help, but you can work without it. Earlier on I found it hard to get proportions right when drawing straight onto the computer, but it gets easier with practice. Everything does. It depends really on how you want to go about doing digital art. Hell, 3D modelling with Unity is digital art.