What textures do they use? What about spacing? How do they mix fonts? What colors do they mix? How do they use perspective and lighting? What's unique about the design?
Then try to design stuff yourself. I've found that doing is the best way to learn, and that's true for more than just design. Study great designers, make your own stuff, and you'll see your skill level gradually improve. I don't consider myself an expert designer but I know how to mix and match things to make a reasonably compelling design. This is how I learnt.
If you want to go beyond that point, and I may be talking beyond my qualifications now, I believe you should pick up the pen and paper and start working on your traditional drawing technique -- get the hang of value and form.
First read some of the basics articles on design basics, typography, layout, color, etc.
Start a scrapbook of designs you like or things that inspire you fashion, scenery, signs, logos, screenshots, web layouts, architecture, etc.
Now try to recreate some of the designs that you like, not as easy as looking at em, but you need the practice. As you get better you will form your strengths and inner-opinions and build your own style to make your own stuff.