Photoshop :: Working With Cutouts

I'm not sure how this is done in Photoshop, and have not yet seen any tutorials on it. I was wondering if someone could provide me with some pointers.

There are two questions for this topic, first I want to describe what I'm working on then I'll ask the questions.

I took a picture of my daughter on her first birthday riding a stuffed horse.
I then found a picture of a horse for sale on the internet and wanted to cut my daughter out and put her on the horse.
I also took a picture out my front door. and I want to put the horse in my front yard.

I'm merely doing this as a self-tutorial as I am trying to learn Photoshop - I don't have any goal with the pictures other than to learn how to accomplish two things.

1) When cutting out the horse, and my daughter for that matter, I have an issue with the hair. In the original picture the horse's tail is dangling over lush grass. I live in the desert - so I either will need to cut out some of the tail or learn how to dissolve the green from the tail. Is there a way to accomplish this in photoshop?

2) The second issue is lighting issues. The original pictures of the horse and my daughter were taken on bright sunny days. The picture in my front yard was taken overlooking a storm coming in from the mountains in the distance. So I need to learn how to blend the images correctly to fit into the stormy picture.