creating a child theme?

I’m trying to modify a couple of things in the Twenty Eleven theme. I’ve read about not modifying the default theme but changing the child theme – How do I create a child theme?
I want to remove two things from the top of the page: the white space (so there is no space between my header image)
and the Search box – this will be a collection of pages, no blog to search.
Once I have a child theme, where do I find the code to change in the stylesheet -which is where I presume the changes have to be made.
Another question – once I’ve created the child theme how do I activate it? I have no WP experience, all of the web development I’ve done, I’ve done with Dreamweaver and am struggling to find my way around WP. So any help is appreciated.