Most Helpful Girl
It's not that girls find sports clothes unattractive, it's just if they wear it all the time. We like when guys put a little effort in how they look. But too much can be a little annoying. Honestly, if you just invest in a pair of simple Nikes, Adidas, or Vans it can help a lot. Pair that with a comfy pair of jeans and a tshirt and you're set. Urban Outfitters may be pricey but they have some pretty cool simple shirts. Also, put a nice smelling cologne on. Just not a lot to the point you can smell the alcohol.
Oh, and the brands they have at Tilly's or any surf/board shop are cool.
You can also try looking up some outfits on lookbook.nu although, some of the outfits may be a bit feminine for your taste.