that's the tricky part, you can be a trend setter (slim chance) or a trend follower (lot higher chance), but you can't even be the latter either without at least a basic knowledge
but why do you want make a fashion account at all? just because it's the trend on ig and can make money?
you would be better of with a niche what you enjoy and have some insight on, even if it's not the most popular niche on ig
or if you're fixed on the fashion niche and you're interested in it at least a bit, you should make yourself familiar with it, there are a lot of fashion blogs, forums, other ig accs, tumblr, reddit or even fourchan have a fashion board, start reading/observing or else i can't really see you succeed