Semi-formal event! What to wear?

I have a semi-formal event tomorrow night, although I just found out it was semi-formal. I'm a girl, and I don't know what to wear! I've made a habit of avoiding anything where I have to dress up, but I have to attend this one and I don't know what to wear. My closet is full of jeans, tanktops and converse. I don't have any dresses, or dress pants even. Not sure how I'll be able to make myself presentable. Any help anyone?