how can i crop a photo 600x600 pixels?

Hello guys

I'm new at aperture and in photo editing so please bare with me a little

I shot some photos of the family for the official use online and they are requiered to be 600x600 pixels. I cant seem to get that via crop tool (to adjust the pixels). When i change the image size the photo proportion is messed up.

Anybody can help with that?