Well,
1 - some sort of cincher, or corset helps give you curves. You can also buy under padding for your butt and theighs. But I think choosing the right outfit does wonders for creating the illusion, just like on women. As young as you are I think you probably don't need to worry about it so much. Youthfulness is very feminine.
2 - Try 'tucking'. There is a topic on it in the wiki on this site. Or google it.