child themes

I am using a child theme for 2012 to make some added css content and images.
I have never done this before and so far things are going sort of OK, all but a couple of small hiccups I'm not sure about.
For one in appearance the enabled child theme is saying:
"This child theme requires its parent theme, Twenty Twelve."
Is this normal? have I missed a step somewhere?
I am also having some difficulty with the CSS seemingly inheriting unwanted values from WP that is throwing out my template layout and div's. Mainly it looks like line-height and margin settings from looking at the code it's using.
This is quite strange and I don't understand it as I can not work out why it would use them if not called for. I Believe it is extremely unlikely that the classes and div ID's would clash in order to call for added line height values that I have not input.
Is there a way to isolate a section of code put into a page that will ignore any WP CSS?

the only other thing I am about to try and think it is OK is adding CSS to the child theme that overrides the main CSS.
For example using the 100% page in 2012 there is a ridiculous amount of space between the header image and where the main body starts - that is only used to tile the page. This is controlled by line-height I think. So if I added the same CSS code into the child theme using a smaller more proportionate spacing for this will it override the main CSS style of the theme?

phew! there's a bit more there that I thought - sorry.