Personality and being charming could win most girls over. Be yourself don't change up for any one but appearance is attractive. Of course, it is what draws people in. ( You should appear like you at least care about what you put on your back) But the most important traits to have are confidence and a intriguing conversation.
I think - more important than wearing clothes which are 'cool' or on trend - it's wearing clothes which suit you.
There's nothing more unattractive, for me at least, when I see a guy dressed a way in which he is uncomfortable just to impress people or make them blend in. Don't get me wrong, there's nothing wrong with dressing up every now and then, or styling his hair slightly ... but nothing sticks out more like a sore thumb than a guy who has changed his style, or compromised his own comfort to fit in.
Obviously there are some people who will disagree. But looking good in something you're comfortable in. I've seen some guys look their best in a pair of old, oil stained jeans, not shaved for a few days and a baggy shirt. And others who have a nicely ironed shirt, styled hair, and neatly fitted trousers.
It's more about how you wear what you wear than the actual clothes or style itself. Confidence, comfort, and a sense of contentment in your personal identity.
As long as they look presentable and take somewhat of a pride in their appearance, to me that's usually all that matters. I mean I won't lie, I do find myself attracted to guys who have nice tshirts or jeans or who dress up a lot (I really like the scene and emo looks), but it's not everything. If you dress nice and have no personality or are a total jerk, it doesn't matter to me what you wear. I will definitely not be attracted to you.
I love guys that wear sweats-they look gorge and were I live you would def get noticed and looked at automatically if you wear sweat pants and hoodies and trainers.
Looks are not as important as the way he dresses though. I've seen guys that would never be on the cover of GQ magazine but they were sexy because they dressed nice and had some swagger about themselves. A man can be ugly and be sexy. And if all else fails, try to get a nice body.
I'm going to be honest and tell you it's pretty important because no matter how hard people try, they always judge others as soon as they come into view. It's a lot harder to get a girl to like you when she isn't physically attracted to you in the beginning, but I'm not saying it's impossible. You will need a lot of confidence when showing her that you on the inside is a lot more stunning then the good looks on other guys. Becoming friends is the first step. That shows she enjoys spending time with you and girls always have second thoughts on whether they like one of their guy friends or not. You will most likely become one of her "options" (I don't know a better word for it). I'm probably confusing you right now, it takes a lot more than a paragraph to explain what goes on in a girl's head. If you have any further questions I am more than happy to help. :)