i’m not sure what you mean by ‘digital photography’. but here goes:
digital photography is essentially the same a regular photography. issues like exposure time, aperture, focus, iso all react the same or similarly in a digital camera as in a film camera. learning those basics of making a good photograph should be your first step.
secondly you will want a way to ‘process’ those digital photographs. first you should learn about the raw file format. it’s a non-destructive file format and requires some special software. it’s a simple process but it’s worth learning to do right.
lastly, you will need to learn photoshop. i recommend ‘photoshop classroom in a book’ for what ever version of ps you have. it’s an excellent book that breaks ps down into manageable lessons. it also comes with a cd with project files that follow the book. this way you don’t have to make your own files and just guess that you’re doing it right.
i’m inlcuding one website that just teaches about photography. this site breaks down basic photography fairly well. scroll down the page a bit to find the lessons.

Look into IrfanView, it is a freeware program and a quite capable one.

Try (The New York Institute of Photography)
I have seen the course material for their film study. It is actually pretty comprehensive. They have added a digital course, I expect it to be about the same. Like any self-study course, you get out what you put in.