So I got asked to Sadies...?

Ok. So I got asked to Sadies by this girl I really like. And it has come to my understanding that you need matching outfits. So over the weekend, we went to the mall and got these Kermit the Frog t-shirts at Spencer's. But some girl told me you need to look "presentable". So I was thinking, would it look nice with a cardigan over it? If so, where can we get cardigans for a girl and a guy? The shirt is all green with Kermits face on it. If not, whats a nice outfit? Thanks!