Just start. ;) There's a lot more to design to just opening Photoshop (which I don't use for designing ironically), but what I've always done is look at how others did/do it.
Check out Dribbble (dribbble.com) for graphic design things, Webdesigner Depot (webdesignerdepot.com) for articles, Patterntap (patterntap.com) for examples and Smashing Magazine (smashingmagazine.com,the design section) for even more articles.
The advantage that you have is that you can code, so designing by coding is pretty nifty.
One last advise, instead of learning Photoshop (which is handy for photos etc.), try learning Illustrator or Inkscape. Know how to make and manipulate vector images (SVG) is getting more and more important and should be one of the skills of a designer (in my opinion).
If you have more specific questions, let me (and us) know!