Let's see... shoe-wise, I'm unsure which brand of running shoe's the best at shock absorbtion, but running shoes in general are probably only good if your character's planning on a duel involving a lot of parkour. Otherwise, their best bet iis probably a pair of hiking boots, if they've got 'em, since those are specifically made to be protective and sturdy, and a good start to their makeshift armor. Plus, if rough terrain is at all a part of your story, such boots will do your character loads of good.
As for clothing, obviously not much in the modern closet will protect against modern weapons, but a few layers might help cushion any blows in a fistfight, and any sort of denim or leather seems like a start to protect against sharp objects and weapons, though the sleeves on a denim jacket could make maneuverability tough in a fight. If your character plays any sports, they may have some protective gear, which would definitely be of use, and if their sport involves a helmet, they'd be very, very wise to wear it.
If your character has any time to plan for whatever they need this "armor" for, though, they'd probably be best off forgoing their own wordrobe and heading out shopping for it instead, since even if they've only got a couple hours to put something together, most towns and cities have a sporting goods store or two that will sell protective gear (such as the attire hockey players wear, which actually does look a little like armor XD ).
Hope that helps!