I won't be able to make you an outfit, but I can offer my opinion. I would not recommend the outfit you suggested. If you're aiming to look older, a T-shirt and a hoodie will not help at all. Since it's a party, I think you should also dress up at least a little above casual. I would have to agree with Lil' Dog's suggestion for an outfit. The sophisticated style of that top along with the collar will make you appear more mature.