Just like programming, business, etc.;

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.