The first step is finding what image you wish to portray, which needess to say is easier said than done.. do you like more gothic lolita stuff, more boho hippy items, or do you steer clear of indie fashion and aim to stick to the trends.. you need to consider this as well as your body type and what clothes suit YOU. It's all very well if you love tight fitting stove-pipe jeans, but if ur morbidly obese, how tasteful is that really gonna look? haha, but you see what I mean.. What colours do you enjoy wearing and what colours suit you? For example, I have blue eyes and try to highlight them so i try not to wear colours that wash them out and I dont enjoy my pale skin, so i avoid black or anything that makes me look vamp, personally.
window-shopping online stores and tumblr posts give you a hint at what exactly you want to aim for style wise