Here is an article on how to dress pop punk these days. During the 90's long sleeve shirts under band tees, with Dickie khaki shorts, with high socks and vans were popular. But these days the style of clothing have changed.
Buy a pair of vans.
Buy some skinny jeans. The guys shouldn't wear skin tight jeans. Pac Sun sells bullhead jeans which have nice fits.
Get some band merchandise. What are your favourite pop punk bands? Buy merchandise from their stores. Band merchandise is a little overpriced but you want to show what bands you listen to.
Go to your local thrift stores and find clothes that are different or "hipster"
Get solid zip up sweatshirts and graphic pullovers.
Slouchy beanies are optional
Get a bunch of bracelets and wear them on either wrist.