I started just buying whatever I could find, very scattershot, and bought some decks that don't see the light of day very often now, decks that I don't consider good decks, or in some cases, even functional.
It'd be great to have someone tell you right off the bat, "Buy this, not that, get those, not these," but you really can't be told, because you have to develop your own tastes and judgment criteria.