Design is a subject to be learned like any other - there's whole books and schools dedicated to it. Go to the library, look through some art and design books, if you see a style you like then write it down and emulate it (don't copy it outright, just use it as a framework or guidance). If you're into what a lot of people here are calling "minimal", look at modernism and the swiss-style. Browse published graphic design poster histories and start to pay attention to how the compositions are put together and what makes styles similar and different, and typography. In no time you'll have a greater appreachiation of the craft and your own designs will look a lot better. It's a lot like programming actually.
