Lots of great tips above, and I can't top the technical advice that has already been presented. I can give you one tip though, from looking through my photo archives: To me, what makes photography really special is its ability to freeze a moment in time. I think this is why my favourite motives are things transient, ephemeral or in motion - wildlife, birds in flight - and children captured at moments when they were unaware of being observed. I guess my best advice is to keep a look out for those split seconds of opportunity - they are better captures than any prearranged composition, to my eye.
The little one in this photo turns four today: This photo is her first encounter with her older brother.