I'm really skinny and flat...?

I'm 14, and I got my period 2 years ago. I'm reaaalllyy skinny. Like, Nicole Richie thin. No matter how much I eat, I can't gain weight. I pretty much sat around all summer eating ice cream, and I did not gain a single pound. I really hate it. I have such a small chest. I just want to be AVERAGE SIZED. I want to be able to wear a strapless dress. And actually be able to fit into stuff when I go to the mall. A lot of woman want to be thin, but there's thin and then there's too thin. And I'm too thin. I have always been underweight, even in 2nd grade. Give me tips for gaining like 20 pounds, please ):