Header text too big & too much white space

Hi there,

First up, apologies if this has been posted before but I can’t seem to find it on here. My page headers are too big on all my pages. Also there is way too much white space between the top header images and the main header text. See site for an example of what I mean:

https://neilatkinsonphotographer.com/

From what I’ve read so far I need to go into the editor and amend the css. Can anyone out there give me the specifics on how to do this. The theme is Twenty eleven.

Thanks in advance.