Graphic Design isn't just 'drawing on a computer'.
If you want to learn computer illustration, that's certainly a viable goal. The best way to learn is to do. Grab a copy of a vector illustration tool (Inkscape is a nice free one, Illustrator the main commercial option) and a raster one (The GIMP is the popular free option, Photoshop the popular commercial option) and get to creating art.
If you're looking to learn the broader concepts of graphic design, then there's a lot of theory and history to go along with your drawing skills. Topics can include:
- color theory
- human factors
- interaction design
- motion design
- user experience
- design history
Google any of the above to find plenty of resources or search through Amazon.
Just a handful of many books to consider:
Think like a Graphic Designer:
Anything by Stephen Heller:
Graphic Design Referenced:
Stop Stealing Sheep: